lavf/id3v2enc: fix covert art display on some software

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

For reference (ID3 metadata):

[ (Apic frames, ...) ]                               -> cover don't show up
[ (Apic frames, ...), (Padding) ]                    -> ok
[ (ID3 frames, ...), (Apic frames, ...) ]            -> cover don't show up
[ (ID3 frames, ...), (Apic frames, ...), (Padding) ] -> ok