add on detach

src/layer/layer.hpp

@@ -16,6 +16,7 @@ namespace VE
 
     public:
       virtual void on_attach() {};
+      virtual void on_detach() {};
       virtual void render() {};
       virtual void update() {};
       virtual void on_event(event e) {};

src/utils/define.hpp

@@ -6,6 +6,7 @@
 #define VE_OVERIDE()\
   public:\
     void on_attach() override;\
+    void on_detach() override;\
     void render() override;\
     void update() override;\
     void on_event(VE::event e) override