We need to implement the show_frame() function from GstVideoSink, which
handles both preroll and normal frames, instead of just
GstBaseSink.render(), which is called only for normal frames.
This was changed for GStreamer 0.10 by 3b20608f.
Change-Id: I4823a575d499cd0d6f9f4cb62e0420e070a05214
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>