This is based on a spec at https://aomediacodec.github.io/id3-emsg/,
further based on ISO/IEC 23009-1:2019.
Within libavformat, timed ID3 metadata (already supported by the
mpegts demuxer and muxer) is handled as a separate data AVStream
with codec type AV_CODEC_ID_TIMED_ID3. However, it doesn't
have a corresponding track in the mov file - instead, these events
are written as separate toplevel 'emsg' boxes.
Signed-off-by: Martin Storsjö <martin@martin.st>
fbd5e238d4