cogl-gst: emit ready signal on sink only after the first frame is uploaded
cogl-gst: emit ready signal on sink only after the first frame is uploaded

Developers listening to the 'ready' signal on CoglGstVideoSink might
call process the current frame once the signal is triggered. We need
to ensure the first frame has been uploaded before letting people know
that the sink is ready.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

djdeath/cogl,djdeath/cogl,djdeath/cogl