Release videoframe on painter stop()

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

This is a backport of change Iaf3eb13eaf51fbc22fab6b1f80db8e8978ac328d
from the qt/qtmultimedia project.

Change-Id: I8ecc7fec585c99e4010496e201b3c3b343f344a1
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>