decoder: h264: track previous reference frames.
decoder: h264: track previous reference frames.

Improve closure of gaps in frame_num by strictly following and trying
to fill them with previous reference frames. So, they are now tracked
thus avoiding insertion of dummy ("greenish") frames.

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