gdk: Coalesce 2 GdkWindow fields in GdkPointerWindowInfo struct

gdk: Coalesce 2 GdkWindow fields in GdkPointerWindowInfo struct

One used to point to the toplevel and the other to the client-side window
that the pointer pointed to. The latter was made to be like the former in
most places, so put those together, and fix the remaining cases where the
variable might not end up with a toplevel/native window.