Fix QSGGeometry object leak

Set QSGNode::OwnsGeometry flag to QSGVideoNode, otherwise the QSGGeometry
object created in QSGVideoNode::setTexturedRectGeometry() will be leaked.

Change-Id: I383b4fe2ff8c7921d2c9c101d4d90fac5a08365a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>