Opus release 1.3

Opus 1.3 is the latest major release of Opus and brings many improvements,
including:
- 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
- Support for ambisonics coding using mappings 2 and 3
- Making it possible to use SILK down to bitrates around 5 kb/s
- Using wideband encoding down to 9 kb/s
- Minor quality improvement on tones
- Security improvements

Notable bug fixes include:
- Fixes to the CELT PLC
- Bandwidth detection fixes
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