lavf/id3v2enc: fix cover art display on some software

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