This is needed for Starship Titanic, where videos can have a secondary
video track. It was simpler to use the callback as a means to select
one video track each across two decoders than trying to make VideoDecoder
and/or AVIDecoder support decoding from multiple video tracks simultaneously
Starship Titanic in particular needs this, since some of the videos
have extra junk at the end of the file, such as ycursors.avi,
and parsing fails if we don't respect the filesize field