implement animation panel interaction

src/canvas_layer.cpp

@@ -227,8 +227,23 @@ bool Layer::create(int width, int height, std::string name)
 bool Layer::add_frame()
 {
     m_frames.emplace_back();
-    m_frame_index = m_frames.size() - 1;
-    return frame().create(w, h);
+    return m_frames.back().create(w, h);
+}
+
+int Layer::total_duration() const noexcept
+{
+    int duration = 0;
+    for (auto& f : m_frames)
+        duration += f.m_duration;
+    return duration;
+}
+
+void Layer::goto_frame(int frame) noexcept
+{
+    int i = 0;
+    for (i = 0; i < m_frames.size() && frame >= 0; i++)
+        frame -= m_frames[i].m_duration;
+    m_frame_index = i - 1;
 }
 
 void Layer::resize(int width, int height)