- 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
- textsubtitlesource, chunked piping
- More efficient MP3 loading when reading sequentially