Adding an arbitrary amount of padding bytes at the end of the
ID3 metadata fixes cover art display for some software (iTunes,
Traktor, Serato, Torq).

For reference (ID3 metadata):

[ Apic frames ]                      -> cover doesn't show up
[ Apic frames, Padding ]             -> ok
[ Apic frames, ID3 frames ]          -> ok
[ ID3 frames, Apic frames ]          -> cover doesn't show up
[ ID3 frames, Apic frames, Padding ] -> ok