win32u: Pass all drivers window rects relative to the virtual screen.

This makes it possible to position client windows relative to their toplevel with the driver window rects only, without having to call into win32u to map window points. Updating the child window states whenever a window is moved will also let us to update the driver client-toplevel surface association in WindowPosChanged, for cases where the ancestor of a client surface is reparented.

This would be useful both for winewayland GL/VK child window support, and to implement virtual display settings, where we need another level of monitor DPI, and requires passing additional parameter to win32u when mapping window points.