gst/matroska/: Add a "vfunc" to the track context for postprocessing frames and conve...

gst/matroska/: Add a "vfunc" to the track context for postprocessing frames and convert the wavpack and subtitle post...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_track_free),
(gst_matroska_demux_add_mpeg_seq_header),
(gst_matroska_demux_add_wvpk_header),
(gst_matroska_demux_check_subtitle_buffer),
(gst_matroska_demux_parse_blockgroup_or_simpleblock),
(gst_matroska_demux_video_caps), (gst_matroska_demux_audio_caps),
(gst_matroska_demux_subtitle_caps):
* gst/matroska/matroska-ids.h:
Add a "vfunc" to the track context for postprocessing frames and
convert the wavpack and subtitle postprocessing to this vfunc.
Copy buffer flags in those functions to the new buffers too.
Parse CodecState elements of Blocks.
Add a postprocessing function for MPEG video that adds the sequence
header from the codec private data or codec state to the frames if
it's not already there.