Texture.sourceItem not correctly supported with RHI

When embedding 2D items into the 3D scene (via a texture, by using the sourceItem property of Texture), like the sourceitem manual test does, the application crashes due to a null command buffer when attempting to render the 2D item layer (as in Qt Quick layer) into a texture.

This is because the QSGDynamicTexture updateTexture() call is made before QQuickWindow::renderSceneGraph() initializes the QSG(Default)RenderContext. Look into how this is supposed to work.