For chapter images, the mov demux produces streams with disposition set
to attached_pic+timed_thumbnails. This patch fixes to properly recognize
streams that should be encoded as cover image (ones with only and only
attached_pic disposition set).
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>