Draft: packetizer: mpegvideo: try to always compute missing pts

PTS is required for reordering captions. ref #27248

Didn't really find a regression in my samples. But relying on frame reference number is not really reliable as mpeg1/2 doesn't use this for DPB management. Could with some PS corner cases with less frequent explicit PTS. (PS doesn't provide DTS/PTS for each frame)