GDK W32: Fix pointer-under-window code for custom resize

GDK W32: Fix pointer-under-window code for custom resize

This code:
>  gdk_window_get_root_origin (window, &x, &y);
>  x -= root_x;
>  y -= root_y;
>  pointer_window = gdk_device_get_window_at_position (device, &x, &y);
was meant to find the child gdk window at coordinates root_x and root_y.

These 4 lines had 2 bugs:
1) x = x - root_x (same for y) is wrong, it should be x = root_x - x
2) gdk_device_get_window_at_position() does not give you the window
   at position x and y. It gives you the window under the device
   (mouse pointer) and the returns the device coordinates in x and y.

https://bugzilla.gnome.org/show_bug.cgi?id=763533