vaapioverlay: ensure sinkpad has current buffer
Use the gst_video_aggregator_pad_has_current_buffer API to check if the current sinkpad has a queued buffer before attempting to obtain a input buffer from the base plugin. If the sinkpad does not have a current buffer, then it is either not producing them yet (e.g. current time < sinkpad start time) or it has reached EOS. Previously, we only handled EOS case. Example: gst-launch-1.0 videotestsrc num-buffers=100 \ ! vaapipostproc ! vaapioverlay name=overlay \ ! vaapisink videotestsrc timestamp-offset=1000000000 \ num-buffers=100 ! video/x-raw,width=160,height=120 \ ! overlay.