use render vmethod instead of show_frame
use render vmethod instead of show_frame

Since GstVideoSink shows the preroll frame and we don't want that,
this commit changes the use of render() vmethod from GstBaseSink
rather than show_frame() from GstVideoSink, to ignore the prerolled
frame.

sreerenjb/gst-checksumsink,sreerenjb/gst-checksumsink