add vr_draw_ui method and make ui observer as method to be wildly available

src/app.h

@@ -137,9 +137,11 @@ public:
     void clear();
     void tick(float dt);
     void update(float dt);
+    bool update_ui_observer(Node* n);
     bool vr_start();
     void vr_stop();
     void vr_draw(const glm::mat4& proj, const glm::mat4& view, const glm::mat4& pose);
+    void vr_draw_ui();
     void async_start();
     void async_update();
     void async_redraw();