Any data that sticks around in a decoder, especially frames that
haven't been retrieved, may cause issues for playback.
This is especially the case with H.264, since its arbitrary frame
ordering to allow reference frames to precede B-frames causes it to
hold onto frames, and causes the playback manager to get back a frame
at a completely wrong timestamp after seeking.
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.
