mpeg2: allocate dummy picture for first field based I-frame.

mpeg2: allocate dummy picture for first field based I-frame.

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.