Opus release 1.3-beta

This is a beta release towards the upcoming 1.3 release. Changes include:
- The spec fixes in RFC 8251 are now enabled by default
- Improvements to the VAD and speech/music classification using an RNN
- Improvements to stereo speech coding at low bitrate
- Added support for ambisonics projection using mapping 3 (disabled by default)
- Fixes to the CELT PLC
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