dnxhd: Support DNx444

Bug-Id: 99
Bug-Id: videolan/9620
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
hevc: Conceal zero td

It was done only in check_mvset(), while mv_scale() is called also by
dist_scale().

Sample-Id: 00001579-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org