add cursor visibility mode

src/node_canvas.h

@@ -5,6 +5,7 @@
 class NodeCanvas : public Node
 {
 public:
+    enum class kCursorVisibility { Never, SmallBrush, NotPainting, Always } m_cursor_visibility;
     std::unique_ptr<Canvas> m_canvas;
     RTT m_blender_rtt;
     RTT m_cache_rtt;
@@ -32,4 +33,6 @@ public:
     void reset_camera();
     void create_buffers();
     void set_density(float d);
+    void set_cursor_visibility(kCursorVisibility mode);
+    void update_cursor();
 };