add template Node::clone() and remove destroy_immediate

src/node_canvas.h

@@ -27,7 +27,7 @@ public:
     virtual void draw() override;
     virtual void handle_resize(glm::vec2 old_size, glm::vec2 new_size, float zoom) override;
     virtual kEventResult handle_event(Event* e) override;
-    virtual void destroy_immediate() override;
+    virtual void destroy() override;
     virtual void on_tick(float dt) override;
     void reset_camera();
     void create_buffers();