Constify the cursor geometry in ::cursor-event

We don't allow changing the cursor geometry inside the ::cursor-event
signal handlers; for starters, it would make binding the signal a
huge mess, and it would also potentially break the whole actor.

clutter/clutter-text.h

@@ -76,10 +76,11 @@ struct _ClutterTextClass
   ClutterActorClass parent_class;
 
   /*< public >*/
+  /* signals, not vfuncs */
   void (* text_changed) (ClutterText           *self);
   void (* activate)     (ClutterText           *self);
   void (* cursor_event) (ClutterText           *self,
-                         ClutterGeometry *geometry);
+                         const ClutterGeometry *geometry);
 
   /*< private >*/
   /* padding for future expansion */