In op_[v]open_url() and op_[v]test_url(), if we successfully
 connected to the URL but fail to parse it as an Opus stream, then
 we would return to the calling application without clearing any
 OpusServerInfo we might have filled in when connecting.
This contradicts the general contract for user output buffers in
 our APIs, which is that they do not need to be initialized prior
 to a call and that their contents are untouched if a function
 fails (so that an application need do no additional clean-up on
 error).
It would have been possible for an application to avoid these leaks
 by always calling opus_server_info_init() before a call to
 op_[v]open_url() or op_[v]test_url() and always calling
 opus_server_info_clear() afterwards (even on failure), but our
 examples don't do this and no other API of ours requires it.

Fix the potential leaks by wrapping the implementation of
 op_url_stream_vcreate() so we can a) tell if the information was
 requested and b) store it in a separate, local buffer and delay
 copying it to the application until we know we've succeeded.

This adds support for accessing any binary metadata at the end of
 the comment header, as first specified in
 <https://tools.ietf.org/html/draft-ietf-codec-oggopus-05>.
It also allows the data to be set and preserves the data when
 doing deep copies.

This includes convenience routines specifying the album gain should
 be applied and for parsing the tag.

In commit 41c29626 I claimed that it was not worth the
 machinery to buffer an extra page to avoid a seek when we have
 continued packet data.
That was a little unsatisfying, considering how much effort we make
 to avoid unnecessary seeking elsewhere, but in the general case,
 we might have to buffer an arbitrary number of pages, since a
 packet can span several.
However, we already have the mechanism to do this buffering: the
 ogg_stream_state.
There are a number of potentially nasty corner-cases, but libogg's
 page sequence number tracking prevents us from accidentally gluing
 extraneous packet data onto some other unsuspecting packet, so I
 believe the chance of introducing new bugs here is manageable.

This reduces the number of seeks in Simon Jackson's continued-page
 test case by over 23%.

This also means we can handle pages without useful timestamps
 (including those multiplexed from another stream) between the last
 timestamped page at or before our target and the first timestamped
 page after our target without any additional seeks.
Previously we would scan all of this data, see that the
 'page_offset' of the most recent page we read was way beyond
 'best' (the end of the last timestamped page before our target),
 and then seek back and scan it all again.
This should greatly reduce the number of seeks we need in
 multiplexed streams, even if there are no continued packets.

We avoid seeking when the seek target lies within the packets
 buffered from the current page.
However, the calculation of the page start time was _adding_ the
 first packet's duration to its end time, instead of subtracting
 it.

When assertions are disabled, nothing else uses header_len.

If the packet where we wanted to start decoding was continued from
 a previous page, and _other_ packets ended on that previous page,
 we wouldn't feed the previous page to the ogg_stream_state.
That meant we wouldn't get the packet we wanted, and would fail with
 OP_EBADLINK (because the starting PCM offset of the first packet we
 did decode would already be after the one we wanted).

Instead, check for continued packet data and feed in an extra page
 to prime the stream state.

Thanks to Simon Jackson for the report and the excellent test case.

OS X 10.5.8 does not define AI_NUMIERCSERV either, so instead of
 trying to enumerate the platforms that don't, just test for the
 value itself.
Patch by Dave Evans at MacPorts.

Fixes #2172

Makes style slightly more consistent.
Also fixes the return code of op_fetch_headers() to make it
 consistently return OP_EBADHEADER if the stream runs out of pages
 after a valid OpusHead packet is found.
Previously, if a valid OpusHead was found, it would return
 OP_ENOTFORMAT if it ran out of pages before finding one without
 its BOS flag set, and OP_EBADHEADER if it ran out of pages after
 finding one without its BOS flag set.

width=height=depth=colors=0 should be allowed, but wasn't thanks to
 some missing parentheses.
Thanks to Lithopsian for the report.

We were skipping the BOS page for non-Opus streams, but never read
 in the next page, causing setup to fail with a duplicate serial
 number validation error.
Thanks to Lithopsian for the report.

It helps to increment the comment count.
Thanks to lithopsian for the report.

Thanks to gmaxwell for the report.

These should use the same macros as the rest of the code.

RFC 6555 "Happy Eyeballs" has a few recommendations for
 implementing dual requests to hosts with both IPv4 and IPv6 DNS
 entries that differ slightly from how we used to do it.
This commit updates things to follow those recommendations.

Otherwise redirects we'll stuff two requests into the same buffer
 if there's a redirect to another https URL via a proxy.

This separates the state for whether pipelining might be supported
 (HTTP 1.1 responses with a Server header) and for whether it's
 been explicitly disabled (Server headers from known-bad servers).
This does a better job with repeated (and possibly contradictory)
 headers.

For many Latin1 characters this still worked correctly by pure luck.
Unfortunately, that included my test case.

It was only guaranteed to work in the seekable case.
Thanks to Radio Stadtfilter 96.3Mhz for sending a stream which
 triggered this failure.

All the actual filters are named op_foo_filter(), so
 op_read_native_filter() looked like another one.
Changing the word order should remove the ambiguity.

Don't know if this actually leads to any better code generation,
 but it may make the code a little clearer.

This makes it clear they don't modify the state, which is a useful
 indicator of what is safe to call from the application decode
 callback.

This is a little easier to read, and allows us to save some
 subtractions and a comparison.

The increasingly-inaccurately named op_shaped_dither16() can serve
 this role instead, without much additional complexity (perhaps
 less, once you consider the required shuffling around of the
 function parameters).

This is to correspond to the feature in opus-tools's opusdec.
opusdec itself probably won't be able to use it, because it still
 wants to do dithering after resampling, but the motivation for the
 feature is the same.

Previously I'd said this would require a large stack buffer, which
 was true if you simply wanted to re-use the existing
 op_short2float_filter() and floating-point op_stereo_filter().
But the latter is not normally compiled for fixed-point anyway, so
 we can instead write new code that doesn't need the stack buffer
 without harming anything.

These were unlikely when I originally expected applications to
 normally provide a decode buffer in the right format of sufficient
 size.
However, the decode filter API works by asking to decode into a
 0-byte buffer, which will trigger both of these tests on every
 packet.
Since this is how three of the four decoding APIs are implemented,
 this is not really that unlikely anymore.

This makes op_fetch_and_process_page() always try to fetch another
 page, like it used to (and like its name implies).
There was only one place where we needed the early return for
 unprocessed packets, and getting rid of that actually simplifies
 the logic there (replacing a conditional continue with an
 unconditional one in a slightly different location).

This is needed to allow advanced usage, like that of opusdec in
 opus-tools, which can simulate packet loss or save the range coder
 state for decoder verification.
It could also be used in a pinch to use libopusfile for access to
 the raw Ogg packets, though this is somewhat of a hack.

Also refactor the other tag functions to centralize the memory
 allocation logic, plus a few other minor cleanups.

It seems somewhat silly to have to write your own strcasecmp to
 satisfy the usage pattern found in opusfile_example: scanning the
 whole tags list and handling certain tags specially (as opposed to
 searching for specific tags, as opus_tags_query() does).
Given that we were already using an equivalent function internally
 to implement opus_tags_query, just expose it directly.

OP_INT32_MIN and OP_INT32_MAX were added in commit 97917914.
This fixes at least one bug, since one of the (extremely unlikely)
 overflow checks had an extra F in it.

This allows the application to report details about the server for
 HTTP[S] streams.
For all HTTP[S], this includes the server software, content-type,
 and whether or not it's using HTTPS.
For live streams, it also includes the station name, description,
 genre, homepage, nominal bitrate, and whether or not it's publicly
 listed.

This was treating every unknown header as a Server header.
Good thing this was last!
The only damage was that we might have enabled pipelining even on
 known-bad servers.

These were only non-static because of oversight.

Surprisingly, the actual initial value of this variable is
 irrelevant, since we always insert 24 bits of data into and then
 read only those bits out.
However, fixing this still removes undefined behavior and removes
 extraneous register dependencies.

- Reject 'not a character' values 0xFFFE and 0xFFFF.
- Remove some unnecessary string length checks.