Fix XShape

Commit c3a04bf unintentionally broke XShape handling. By studying the code
extremely carefully, I found this inconsistency with the code that was
there before.

https://bugzilla.gnome.org/show_bug.cgi?id=635268

src/ui/frames.c

@@ -880,7 +880,7 @@ apply_cairo_region_to_window (Display        *display,
 
   XShapeCombineRectangles (display, xwindow,
                            ShapeBounding, 0, 0, rects, n_rects,
-                           ShapeSet, YXBanded);
+                           op, YXBanded);
 
   g_free (rects);
 }