Decide how to deal with artist vs sort artist problem in mp4 files

[ravicious]

I have an album bought in iTunes Store in which the regular artist/album/title fields are blank and instead the metadata is stored in the "sort" type of fields: sort artist, sort album, sort title. Idk if it's regular iTunes behavior, but we need to come up with a way to handle this.

mp4parse has a list of atoms which correspond to those sort fields.