- onHeader, etc callbacks for Matroska
- https://github.com/Vanilagy/mp4-muxer/issues/83 tell him it's possible now
- is this fixed? https://github.com/Vanilagy/webm-muxer/issues/50
- cross-track offset for streaming sources
- configurable fragmented mp4 fragment size, like the mp4-muxer PR
- fix rotation matrix thing, make sure its counter-clockwise everywhere (is this a breaking change from mp4-muxer?)
- canvassink ring buffer