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
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