In P-pictures, prediction shall be made from the two most recently
decoded reference fields. However, when the first I-frame is a field,
the next field of the current picture could be a P-picture but only a
single field was decoded so far. In this case, create a dummy picture
with POC = -1 that will be used as reference.
Some VA drivers would error out if P-pictures don't have a forward
reference picture. This is true in general but not in this very specific
initial case.
Introduce a POC field in GstVaapiPicture so that to store simpler sequential
numbers. A signed 32-bit integer should be enough for 1 year of continuous
video streaming at 60 Hz.
Use this new POC value to maintain the DPB, instead of 64-bit timestamps.
This also aligns with H.264 that will be migrated to GstVaapiDpb infrastructure.
