mpeg2: optimize scan for the second start code.
mpeg2: optimize scan for the second start code.

Optimize scan for the second start code, on the next parse() call so that
to avoid scanning again earlier bytes where we didn't find any start code.

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