qml: proper initialization if scene is already initialized
qml: proper initialization if scene is already initialized

The scene graph can be initialized when the we receive window handle change
notification and so we will not receive a scenegraph initialization
notification.  Initialize ourself in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=758337

GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good