mpegvideo: fix invalid picture unreferencing.

Mpegvideo would free frames as soon as they're not the next or prev
picture. This is fine for a single-threading model, but fails miserably
in a system where pictures can be referenced (as e.g. last/prev pic)
in other threads. Keeping track of ownership of pictures keeps image
references (e.g. motion vectors, or the reference of a motion vector)
alive as long as the picture data itself is alive.

This also happens to fix make THREADS=[3-16] fate-vsynth[12]-error.
