Don't mix up a picture's type with its description.

Music::Tag::FLAC reads a picture's description and sets it as the picture type.  Actually these are different fields (see https://xiph.org/flac/format.html#metadata_block_picture).

This has started catching me out because Musicbrainz now allow people to set a free-text comment on each image.  This gets tagged as the picture's description, causing Music::Tag::FLAC to report nonsensical picture types.

The following patch separates these out.