sourceItem texture content rendering must not be deferred

Run the sourceItem manual test or watch the QtWS talk draft video. Any time a Texture with sourceItem is used, the Quick content does not appear right away. Rather, the textured object appears white, and gets textured correctly with the Quick content only a number of frames (1? 2? 3?) later.

There may be legitimate reasons why the rendering of the QSGLayer cannot be done immediately, but it should be investigated once again what we could do about this.