add frame duplicate

src/canvas_layer.h

@@ -23,6 +23,7 @@ struct LayerFrame
     bool create(int width, int height, int duration = 1);
     bool resize(int width, int height);
     void clear(const glm::vec4& c);
+    LayerFrame clone() const noexcept;
 };
 
 class Layer
@@ -62,6 +63,7 @@ public:
     bool create(int width, int height, std::string name);
     bool add_frame();
     void remove_frame(int frame);
+    void duplicate_frame(int frame);
     int total_duration() const noexcept;
     void goto_frame(int frame) noexcept;
     void clear(const glm::vec4& c, int frame = -1);