Release videoframe on painter stop()

QVideoSurfaceGLPainter/QVideoSurfaceGlslPainter needs
to release the video frame on stop (just like
QVideoSurfaceGenericPainter already does).

Change-Id: Iaf3eb13eaf51fbc22fab6b1f80db8e8978ac328d
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>