[opus] Many changes landing in master

[Jean-Marc Valin]

Hi,

I have just landed all of the changes on the exp_lbr_tune branch into
master. I already did a lot of testing on the exp_lbr_tune branch, but
considering there are more than 60 commits landing at once, it's still
possible that something will break for some setups. Please give master a
try and report any problems or regressions with it.

Among improvements that just landed are:
- SILK/hybrid CBR improvements at lower bitrates
- Low bitrate hybrid mode improvements (16 - 24 kb/s)
- Implementing DTX for CELT-only modes
- Better coding decisions when FEC is enabled
- Better bandwidth switching decisions
- General quality improvements to SILK
- Better music quality at low bitrate (32-48 kb/s)

There's still more improvements to come and the result will be released
as version 1.2 when it's ready.

Cheers,

	Jean-Marc