GStreamer: flush the current frame when stopping a media player.

GStreamer: flush the current frame when stopping a media player.

When stopping, we don't actually stop the GStreamer pipeline, we just
pause it and prevent preroll frames from being shown.
We also need to make sure the last presented frame is cleared in that
case, otherwise it stays on screen.
Fixed for both 0.10 and 1.0.

Change-Id: Ibe26a7567f271ae0c3d8819eb9d35d6a95da1c6a
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>