add cursor visibility mode

src/canvas.h

@@ -175,6 +175,10 @@ public:
         if (I->on_mode_changed)
             I->on_mode_changed(prev, mode);
     }
+    template <class T> static T* get_mode(int index = 0)
+    {
+        return dynamic_cast<T*>(modes[(int)I->m_current_mode][index]);
+    }
 
     std::vector<LayerFrame::Snapshot> m_layers_snapshot;