[3.0.x] On the road to 3.0.21

Prepare for a 3.0.21 release, so far we have:

Decoders:

• Improve Opus ambisonic support
• Fix some ASS subtitle rendering issues

Audio Output:

• Fix regression on macOS causing crashes when using audio devices with more than 9 channels

Contrib:

• Update FFmpeg to 4.4.4

If anyone needs something backported, please do it asap and mention it here if it is blocking for the release.