decoder: handle decode-only frames in raw API mode.
decoder: handle decode-only frames in raw API mode.

Fix gst_vaapi_decoder_get_surface() to only return frames with a valid
surface proxy, i.e. with a valid VA surface. This means that any frame
marked as decode-only is simply skipped.

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