mpeg2: fix decoding of multiple slices with same slice_vertical_position.
mpeg2: fix decoding of multiple slices with same slice_vertical_position.

6.3.15 says that "some slices may have the same slice_vertical_position,
since slices may start and finish anywhere". So, we can't submit the current
picture to the HW right away since subsequent slices would be missing.

sreerenjb/sree-gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,sreerenjb/sree-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,sreerenjb/sree-gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi