Label is not read properly for MP4 tracks

[echarlie]

m4a can't be played by johnny-six (see wuvt/johnny-six#18). We should not validate such tracks.

[echarlie]

Well, we're not reading metadata out of these files correctly anyway. c.f. quodlibet/mutagen#480

[echarlie]

okay. mutagen handles this weird. mutagen.easymp4 will give out keys of:

['title', 'album', 'artist', 'albumartist', 'date', 'copyright', 'albumartistsort', 'artistsort', 'musicbrainz_artistid', 'musicbrainz_trackid', 'musicbrainz_albumid', 'musicbrainz_albumartistid', 'musicbrainz_albumstatus', 'musicbrainz_albumtype', 'tracknumber', 'discnumber'],

while mutagen.mp4 gives:

dict_keys(['©nam', '©ART', '©alb', 'trkn', 'disk', '©day', '----:com.apple.iTunes:MusicBrainz Album Release Country', '----:com.apple.iTunes:originalyear', '----:com.apple.iTunes:~length', '----:com.apple.iTunes:SCRIPT', '----:com.apple.iTunes:CATALOGNUMBER', '----:com.apple.iTunes:MusicBrainz Album Type', '----:com.apple.iTunes:ARTISTS', '----:com.apple.iTunes:MusicBrainz Album Status', '----:com.apple.iTunes:originaldate', '----:com.apple.iTunes:ASIN', '----:com.apple.iTunes:ISRC', '----:com.apple.iTunes:MEDIA', '----:com.apple.iTunes:LABEL', '----:com.apple.iTunes:MusicBrainz Album Id', '----:com.apple.iTunes:MusicBrainz Album Artist Id', '----:com.apple.iTunes:MusicBrainz Artist Id', '----:com.apple.iTunes:MusicBrainz Release Group Id', '----:com.apple.iTunes:MusicBrainz Track Id', '----:com.apple.iTunes:MusicBrainz Release Track Id', 'covr', 'aART', 'soaa', 'soar', 'cprt'])

This handling is annoyingly inconsistent (my vote is to adopt pytaglib instead, which provides a reasonably consistent interface).

[mutantmonkey]

I've repurposed this into the issue of label not being read correctly (by mutagen) for tracks in an MP4 container.