fix layer delete

engine/canvas.h

@@ -94,6 +94,7 @@ public:
     Canvas() { I = this; }
     bool create(int width, int height);
     void resize(int width, int height);
+    void layer_remove(int idx);
     void layer_add(std::string name);
     void layer_order(int idx, int pos);
     void stroke_start(glm::vec2 point, float pressure, const ui::Brush& brush);