gst/matroska/: Add Real[Audio|Video] support to Matroska containers.

gst/matroska/: Add Real[Audio|Video] support to Matroska containers.

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_send_event),
(gst_matroska_demux_video_caps), (gst_matroska_demux_audio_caps):
* gst/matroska/matroska-mux.c:
(gst_matroska_mux_video_pad_setcaps),
(gst_matroska_mux_audio_pad_setcaps), (gst_matroska_mux_finish):
Add Real[Audio|Video] support to Matroska containers.
It works fine for:
* decoding real audio/video streams contained in mkv
* 'transmuxing' real (.rm) files into .mkv files
It will not work though for encoding real[audio/video] streams that
don't contain the 'mdpr_data' extra data on the caps.
The reason why this will not work is because I never intended to
duplicate virtually all the 'mdpr' block creation into mkvmux.
Fixes #536067