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.
This should halve the size of frames in memory for frames with 8-bit
color components, which is the majority of videos.
Calculation of the size of subsampled planes has also been consolidated
into a struct. There are likely some places that will still need to
change over to this, but it should prevent issues due to differing
handling of rounding/ceiling.
BT.2020 will mainly be used with bit depths greater than 8, so having
this specialization is mostly pointless until we use fixed-point math
for higher bit depths.
