We don't allow *encoding* a stereo stream with a mono encoder
because that's just silly.

Adds a CELT_GET_LOOKAHEAD() ctl() call instead. Other uses of
celt_mode_info() should not be needed anymore.

These were used because the entropy coder originally came from
 outside libcelt, and thus did not have a common type system.
It's now undergone enough modification that it's not ever likely to
 be used as-is in another codec without some porting effort, so
 there's no real reason to maintain the typedefs separately.
Hopefully we'll replace these all again somedate with a common set
 of Opus typedefs, but for now this will do.

This fixes an issue caused by commit 6c8acbf1, which moved the
 ec_ilog() prototype from entcode.h to ecintrin.h, where the
 ec_uint32 typedef was not yet available.
Thanks to John Ridges for the report.

We were trying to normalize bands that didn't actually exist (e.g.,
 the last band with 320-sample frames at 32kHz).
Thanks to John Ridges for the report.

Mark _BitScanReverse as an intrinsic, as this is not done by default
 in _DEBUG mode (thanks to Bjoern D. Rasmussen for the suggestion).
Move the declaration of ec_ilog() to where the macro using it is
 defined, to ensure it is available when the macro is used.
Thanks to John Ridges for the report.

The return code has been eliminated in favor of using the existing
 error flag in the ec_enc struct.
This also adds the declaration to the header, which JM's importation
 of Koen's adaptation of my original code in b3c05b6b omitted.

Correct a number of uninitialized value problems when end!=nbEBands. Now 8khz/20ms audio can be decoded at 48kHz and vice versa (with the PF turned off).

Change the bit-stream (without causing desync)

Got authorization from all copyright holders

CELT_SET_END_BAND_REQUEST in the decoder was performing the wrong bounds check and not allowing a change to fullband.