When multiple pictures are embedded in a file, the FLAC demuxer
prioritizes them based on the picture type.
This ports the same algorithm over so it can be used by Vorbis and Opus.
It also extends its behavior for FLAC to work with both pictures
embedded in VORBIS_COMMENT blocks and in normal PICTURE blocks.

This also plugs a memory leak in vorbis_ParseComment() when parsing
METADATA_BLOCK_PICTURE tags.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>