add animation playback

src/canvas.h

@@ -188,6 +188,7 @@ public:
     void layer_merge(int source_idx, int dest_idx);
     int anim_duration() const noexcept;
     void anim_goto_frame(int frame) noexcept;
+    void anim_goto_next() 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);