Add gdk_window_get_unscaled_size

This is required for the X backend GL integration. If the
window has a height that is not a multiple of the window scale
we can't properly do the y coordinate flipping that GL needs.
Other backends can ignore this and use the default implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=739750

gdk/gdkinternals.h

@@ -375,7 +375,9 @@ void       _gdk_window_update_size       (GdkWindow      *window);
 gboolean   _gdk_window_update_viewable   (GdkWindow      *window);
 GdkGLContext * gdk_window_get_paint_gl_context (GdkWindow *window,
                                                 GError   **error);
-
+void gdk_window_get_unscaled_size (GdkWindow *window,
+                                   int *unscaled_width,
+                                   int *unscaled_height);
 
 void       _gdk_window_process_updates_recurse (GdkWindow *window,
                                                 cairo_region_t *expose_region);