matroskadec: export cover art correctly

Generally, libavformat exports cover art pictures as video streams with
1 packet and AV_DISPOSITION_ATTACHED_PIC set. Only matroskadec exported
it as attachment with codec_id set to AV_CODEC_ID_MJPEG.

Obviously, this should be consistent, so change the Matroska demuxer to
export a AV_DISPOSITION_ATTACHED_PIC pseudo video stream.

Matroska muxing is probably incorrect too. I know that it can create
broken files with an audio track and just 1 video frame when e.g.
remuxing mp3 with APIC to mkv. But for now this commit does not change
anything about muxing, and also continues to write attachments with
AV_CODEC_ID_MJPEG should the muxer application have special knowledge
that the Matroska is broken in this way.

Fixes trac #4423.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 511585ce7f7272e5069ef011d6be5f073d268901)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

0 files changed, 0 insertions, 0 deletions