lavc: restore copy_block{4,16} functions

They were removed in 9e31729d692f1e721b7ed1a3a0f51b68c064d68f.
PAF demuxer and decoder

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
fate: add PAF audio and video tests

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>