This was predictably nightmarish, given how ridiculous mpeg4 is.
I had to stare at the cuvid parser output for a long time to work
out what each field was supposed to be, and even then, I still don't
fully understand some of them. Particularly:
vop_coded: If I'm reading the decoder correctly, this flag will always
be 1 as the decoder will not pass the hwaccel any frame
where it is not 1.
divx_flags: There's obviously no documentation on what the possible
flags are. I simply observed that this is '0' for a
normal bitstream and '5' for packed b-frames.
gmc_enabled: I had a number of guesses as to what this mapped to.
I picked the condition I did based on when the cuvid
parser was setting flag.
Also note that as with the vdpau hwaccel, the decoder needs to
consume the entire frame and not the slice.
