decoder: mpeg4: fix picture decoder return value for skipped frames.
decoder: mpeg4: fix picture decoder return value for skipped frames.

The picture decoder should return GST_VAAPI_DECODER_STATUS_DROP_FRAME
when a frame should be skipped, so the stream processing is not stalled.

https://bugzilla.gnome.org/show_bug.cgi?id=733324

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