H.264 in Matroska can have blocks with unordered timestamps. Without
passing these as the presentation timestamp into the FFmpeg decoder,
the frames will not be returned in chronological order.
VideoFrame will now include a timestamp that is used by the
PlaybackManager, rather than assuming that it is the same timestamp
returned by the demuxer.
VP9 continues to function, but this also allows AV1 to be decoded. With
this commit, H.264 is still non-functional, as the decoder requires
some extra initial data from the track definition in the Matroska file.
