js: Explicitly dispose all cairo contexts

Due to limitations and bugs in SpiderMonkey's GC, wrapper objects
for cairo contexts and similar may not get cleaned up immediately
after repainting, leading to leaking memory. Explicitly disposing
of such objects after they're not needed can clean up large portions
of memory for cairo surfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=685513

js/ui/boxpointer.js

@@ -388,6 +388,8 @@ const BoxPointer = new Lang.Class({
             cr.setLineWidth(borderWidth);
             cr.stroke();
         }
+
+        cr.$dispose();
     },
 
     setPosition: function(sourceActor, alignment) {