improve animation panel

src/canvas_layer.h

@@ -61,6 +61,7 @@ public:
     void resize(int width, int height);
     bool create(int width, int height, std::string name);
     bool add_frame();
+    void remove_frame(int frame);
     int total_duration() const noexcept;
     void goto_frame(int frame) noexcept;
     void clear(const glm::vec4& c, int frame = -1);