improve animation panel

src/canvas.h

@@ -112,7 +112,7 @@ 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;
+    int m_anim_frame = 0;
     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));
@@ -189,6 +189,7 @@ public:
     int anim_duration() const noexcept;
     void anim_goto_frame(int frame) noexcept;
     void anim_goto_next() noexcept;
+    void anim_goto_prev() 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);