AV1/AV2 uncompressed header changes (meaningful for vk)

 - 16 DPB slots total, not 8: refresh_frame_flags is up to 16 bits, existing_frame_idx up to 4 bits
 - No LAST/GOLDEN/ALTREF/LAST2/ALTREF maddness, refs are a ranked list (refidx[n]), up to 8 refs possible for a frame
 - Implicit ref selection, no longer signalled by header, done in get_ref_frames() during OBU parsing
 - Long-term refs exist now
 - Low impact from PTS != DTS. Just a show_implicit per-frame flag needed, plus same logic from H264
 - saved order hint maddness persists as-is
 - No superres