H.264 in Matroska can have blocks with unordered timestamps. Without
passing these to the decoder when providing data, the decoder will be
unable to reorder the frames to presentation 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.
(cherry picked from commit f6a4973578c4692f33283aee1c089afd0cdae508)
This change is in preparation for implementing audio codecs into the
library and using audio as timing for video playback.
(cherry picked from commit 7c10e1a08d7a109b63c9258578eb98aa9dcc1425)
