The QAbstractVideoSurface was started only when calling play(), causing
frames received before (e.g. when pausing from stopped state) to not
be rendered.
The surface is now started as soon as a media type is agreed between
the EVR and the upstream element. Conversely, the surface is stopped
whenever the media type is cleared.
Change-Id: Ia96a07dbd277adce67de5a9cfbf9acc0d33b6497
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>