Extend _gdk_windowing_window_at_pointer to be able to get toplevels only

Extend _gdk_windowing_window_at_pointer to be able to get toplevels only

This has two advantages:
1) In many backends, this is faster as we can terminate the window
hierarchy traversal earlier
2) When used in gdkdisplay.c::get_current_toplevel() to get the
current toplevel that has the pointer we now correctly return
a toplevel with the pointer in it where the pointer is inside
some foreign subwindow of a toplevel window.

The second advantage fixes some bugs in client side event generation
when the pointer is inside such a foreign child window.