improve animation panel

src/canvas.cpp

@@ -1358,6 +1358,12 @@ void Canvas::anim_goto_next() noexcept
     anim_goto_frame((m_anim_frame + 1) % anim_duration());
 }
 
+void Canvas::anim_goto_prev() noexcept
+{
+    int k = anim_duration();
+    anim_goto_frame((m_anim_frame - 1 + k) % k);
+}
+
 void Canvas::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)
 {