And missing prototypes

Untested

Not so much for old machines, as for getting decent performance
when not setting -march= (SSE2 is part of the amd64 ABI).

Should be able to handle all previous GRU variants and more.

Should work for both CBR and VBR. In the CBR case, we can make
CELT VBR and use DRED to fill the rest.

If configuring CELT for CBR but controlling the bitrate with
OPUS_SET_BITRATE rather than nbCompressedBytes, then the range
coder buffer would never get resized. AFAICT this could never
be triggered in Opus because CBR was also controlled by
nbCompressedBytes.

CELT encoding would just fail when setting CELT to CBR in hybrid mode.
It was never a problem because hybrid CBR was always used with
OPUS_BITRATE_MAX.

Avoids interactions with redundancy settings

SET_BITRATE is now the total bitrate again

We weren't reserving enough bytes for the DRED extension

The bits we don't use won't be wasted, so it's less important to
get exactly the optimal number of bits below the cap.

The gain*2 when overshooting was too aggressive and the undershoot
case wasn't aggressive enough. This now seems to work reasonably well.

Don't attempt to run the neural PLC on the side channel since
we only have one state.

Also, don't code DRED that's redundant with the main packet

Should make synthesis easier in the future