add history to layer create, delete, move, rename, and merge

src/canvas.h

@@ -94,9 +94,8 @@ public:
     std::unique_ptr<Stroke> m_current_stroke;
     std::unique_ptr<Stroke> m_dual_stroke;
     bool m_show_tmp = false;
-    std::vector<std::unique_ptr<Layer>> m_layers;
+    std::vector<std::shared_ptr<Layer>> m_layers;
     Layer m_layers_merge;
-    std::vector<int> m_order;
     std::vector<glm::vec2> m_plane_shape[6]; // screen space projection of the plane
     glm::mat4 m_plane_unproject[6] = SIXPLETTE(glm::mat4(1));
     glm::vec3 m_plane_dir[6] = SIXPLETTE(glm::vec3(0));
@@ -164,7 +163,7 @@ public:
     bool create(int width, int height);
     void resize(int width, int height);
     void layer_remove(int idx);
-    void layer_add(std::string name, std::unique_ptr<Layer> layer = nullptr, int index = 0);
+    void layer_add(std::string name, std::shared_ptr<Layer> layer = nullptr, int index = 0);
     void layer_order(int idx, int pos);
     void layer_merge(int source_idx, int dest_idx);
     void stroke_start(glm::vec3 point, float pressure);