Hm, why just copy front cover PICTUREs from flac to mp3? The flac

Hm, why just copy front cover PICTUREs from flac to mp3?  The flac
PICTURE metadata block has the exact same metadata as the mp3 ID3v2
APIC frame; just copy them all.

flac.c:
    (flac_pictures): Return fields in the same order they're found in
    the FLAC format.  Convert the description to a unicode object
    rather than returning a UTF-8 encoded str object.

taglib.cc:
    (taglib_apic): Take a list of picture tuples and add them all as
    APIC frames.

    (add_apic_frame_to_mp3): Take the list of picture tuples and pass
    them straight to taglib_apic.

flac2mp3:
    (flac2mp3): Take a list of picture tuples instead of just one;
    pass them all to add_apic_frame_to_mp3.

    (find_pics): Rename from find_pic.  Save all PICTURE metadata
    blocks rather than just saving the first front cover picture.