GTK+ 3: fix client->global in a frame with a menu bar

It seems that the right answer was produced in GTK+ 2 due to a
client-location callback, but change the code to use the intended
approach of tracking the menu height.

gui-lib/mred/private/wx/gtk/frame.rkt

@@ -163,7 +163,7 @@
 
     (inherit get-gtk set-size
              pre-on-char pre-on-event
-             get-client-delta get-size
+             get-stored-client-delta get-size
              get-parent get-eventspace
              adjust-client-delta
              queue-on-size)
@@ -463,7 +463,7 @@
     (define/override (internal-client-to-screen x y)
       (gtk_window_set_gravity gtk GDK_GRAVITY_STATIC)
       (let-values ([(dx dy) (gtk_window_get_position gtk)]
-                   [(cdx cdy) (get-client-delta)])
+                   [(cdx cdy) (get-stored-client-delta)])
         (gtk_window_set_gravity gtk GDK_GRAVITY_NORTH_WEST)
         (set-box! x (+ (unbox x) (->normal (+ dx cdx))))
         (set-box! y (+ (unbox y) (->normal (+ dy cdy))))))

gui-lib/mred/private/wx/gtk/window.rkt

@@ -834,6 +834,8 @@
 
     (define/public (get-client-delta)
       (values 0 0))
+    (define/public (get-stored-client-delta)
+      (values client-delta-w client-delta-h))
 
     (define/public (warp-pointer x y)
       (define xb (box x))