Fixed QSGVideoNodes not deleting their materials.

Change-Id: Ia7597ade3066c23c54354e2cc36c907e6bf0dfea
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

src/imports/multimedia/qsgvideonode_rgb.cpp

@@ -249,6 +249,7 @@ public:
 QSGVideoNode_RGB::QSGVideoNode_RGB(const QVideoSurfaceFormat &format) :
     m_format(format)
 {
+    setFlag(QSGNode::OwnsMaterial);
     m_material = new QSGVideoMaterial_RGB(format);
     setMaterial(m_material);
 }