lazy stroke preview rendering

src/node.h

@@ -132,6 +132,8 @@ public:
     glm::vec4 m_clip_uncut{ 0, 0, 0, 0 };
     std::string m_name;
     bool m_display = true;
+    // it's actually rendering
+    bool m_on_screen = false;
 
     Node(const Node&) = delete;
     Node& operator=(const Node&) = delete;
@@ -251,6 +253,7 @@ public:
     virtual kEventResult on_event(Event* e);
     virtual kEventResult handle_event(Event* e);
     virtual void handle_resize(glm::vec2 old_size, glm::vec2 new_size);
+    virtual void handle_on_screen(bool old_visibility, bool new_visibility);
     virtual void handle_parent_resize(glm::vec2 old_size, glm::vec2 new_size);
     virtual void create();
     virtual void init();