mxf: Handle negative edit_rate

Default to 1/25.
mxf: Read field dominance flag

And export the information in field_order.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
hevc: Consider first quantization group any reference to 0, 0

According to my understanding of T-REC-H.265-2013044 chapter 8.6.1.

Sample-Id: 00001438-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
hevc: Reject impossible slice segment

A dependent slice cannot have address 0.
Prevent an out of array bound load in ff_hevc_cabac_init().

Sample-Id: 00001406-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org