Opus release 1.2-rc1

This is the first release candidate for the upcoming Opus 1.2 release. If no
issues are found with it, it will soon become 1.2-final, so I encourage
everyone to give it a try. Changes compared to 1.2beta include:
- Improves quality on files with powerful tones that cause MDCT leakage
- Improves bit allocation on mode transitions (CELT to/from SILK/hybrid)
- More ARM Neon optimizations
- Fixes to the speech/music detection at the very beginning of files
- Fixes to the unit tests (fixes illegal instructions and --disable-static)
This tag has no release notes.