Add option to use TextureView instead of SurfaceView when child windows present

Qt for Android uses QtSurface, a SurfaceView subclass to get a Surface to render on, with one SurfaceView per window. However, due to how SurfaceView is implemented control over z-ordering between the windows is limited - they are stacked in the order in which the Surfaces are created. TextureView would offer more control over z-order, as well as have support for things like clipping, rotation and alpha.

Test drive using TextureView when child windows are present, while keeping the old SurfaceView approach for single top-level windows.