ffmpeg_tag_internal() does not look for a few tags that mpd
supports. Most noteably:

 comment -> TAG_ITEM_COMMENT -> Description
 genre -> TAG_ITEM_GENRE -> WM/Genre (not WM/GenreID)
 year -> TAG_ITEM_DATE -> WM/Year

I *think* that this is the last of the tags that AVFormatContext() in
ffmpeg supports that mpd also uses.