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.