add event target

src/layer/layer.hpp

@@ -22,10 +22,10 @@ namespace VE
       virtual void render() {};
       virtual void on_event(event e) {};
 
-      void set_event_fn(const event_fn<event&>& fn) { execute = fn; };
+      void set_event_fn(const event_fn<event>& fn) { execute = fn; };
 
     protected:
-      event_fn<event&> execute;
+      event_fn<event> execute;
   };
 }