Saves a copy in the decoder

Should be less confusing for profiling

This is a tentative fix for a bug found in fuzzing where the encoder
switched from mono to stereo while in the process of changing bandwidth.
The result was that the newly added side would use the new sampling
rate, while the mid hadn't switched yet, causing an encoder/decoder
mismatch. The fix is that the side rate selection gets overridden
to use the mid rate.
The bug would occur when compiling with fuzzing enabled and using:
./test_opus 0 48000 2 24000 input.sw output.sw

to mono state transition in the same manner as the encoder.

asserts to reflect the actual limits. Update the cwrs test to cover
the complete n,k range used by opus/opus_custom.

During SILK->CELT transitions, we were delaying the mode change by
one frame, but only after having ensured that bandwidth and mode were
consistent. We now do the delaying earlier.

Repacketizer now seems to work, though it doesn't yet handle
unmergeable packets. Also, test_opis no longer attempt to do proper
gapless at the end of the file, which was causing problems for
repacketization, but also with 32 bit overflows

Still more bugs to find

Remove a redundant negative length check in celt_decode.

Was previously testing only far on the negative side and using
the wrong scaling factor

their usage and to make any toolchain that doesn't like to #include
.c file happy.

Previously the autotools build litered whatever directory
'make' was invoked from with intermediate files. Adding
'subdir-objects' to AUTOMAKE_OPTIONS tells it to put them
in subdirectories according to the relative paths to the
source files, .i.e. silk/schur.o for $(srcdir)/silk/schur.c

The change to autogen.sh is necessary to handle the
(apparently new) case where an AUTOMAKE_OPTIONS line is
given without a minimum version number.

Thanks to Tim Terriberry for pointing out the issue.

This better distinguishes the reference implementation,
which is what this string is meant to do. Thanks to
Greg Maxwell for the suggestion.

This lets us verify this part of the API is working.

Remove trailing whitespace, convert tabs to 4 spaces, re-align
test_opus to use 4-space indents instead of 3, and re-wrap some
long lines.

This lets users print it out without context, which is a little
easier for clients.

Rather than compiling in the fixed version number from the configure
file, it's more helpful for testing and debugging to directly embed
the repository's revision. We use 'git describe --tags' for this, since
for tagged revisions it just returns the tag name, and in other cases
is more readable than a raw commit id.

If git isn't present or can't find a repository to query, we fall back
to the static version number in the configure script.

The new opus_get_version_string() call just returns a static string
defined by OPUS_VERSION, which is passed in from the build system
through config.h (or a custom compile line). Provide a fallback to
"unknown" if the build system fails to actually provide that definition.

This restores compilation with Makefile.draft.

In general, this means there will be builds out there with non-specific
version strings, since ports won't be forced to update the string. While
that's unfortunate, I think it's more valuable that the library be simple
to build.