implement animation panel interaction

src/canvas.h

@@ -108,6 +108,7 @@ public:
     std::unique_ptr<Stroke> m_dual_stroke;
     bool m_show_tmp = false;
     std::vector<std::shared_ptr<Layer>> m_layers;
+    int m_anim_frame = 1;
     Layer m_layers_merge;
     std::vector<glm::vec2> m_plane_shape[6]; // screen space projection of the plane
     glm::mat4 m_plane_unproject[6] = SIXPLETTE(glm::mat4(1));
@@ -176,10 +177,13 @@ public:
     bool create(int width, int height);
     void resize(int width, int height);
     Layer& layer() { return *m_layers[m_current_layer_idx]; }
+    std::shared_ptr<Layer> layer_with_id(uint32_t id);
     void layer_remove(int idx);
     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);
+    int anim_duration() const noexcept;
+    void anim_goto_frame(int frame) noexcept;
     void flood_fill(int layer, int plane, std::vector<glm::ivec2> pos, FloodData& plane_data,
         float threshold, glm::vec4 dest_color, std::unique_ptr<glm::vec4>& source_color);
     void stroke_start(glm::vec3 point, float pressure);