Instead of using a single track entry for all blocks in the file, use a
lookup to get the info needed to calculate the timestamp for the
specific track a block belongs to. No change in behavior for
SampleIterator, since that only returns blocks from the track that was
passed. This will be useful for MSE, since it demuxes all tracks at
once.
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.
