add frame duplicate

src/canvas_layer.cpp

@@ -244,6 +244,11 @@ void Layer::remove_frame(int frame)
     m_frame_index = glm::clamp(m_frame_index, 0, (int)m_frames.size() - 1);
 }
 
+void Layer::duplicate_frame(int frame)
+{
+    m_frames.insert(m_frames.begin() + frame + 1, m_frames[frame].clone());
+}
+
 int Layer::total_duration() const noexcept
 {
     int duration = 0;
@@ -436,3 +441,18 @@ void LayerFrame::clear(const glm::vec4& c)
         glClearColor(cc[0], cc[1], cc[2], cc[3]);
     });
 }
+
+LayerFrame LayerFrame::clone() const noexcept
+{
+    LayerFrame dup;
+    dup.m_duration = m_duration;
+    dup.w = w;
+    dup.h = h;
+    for (int i = 0; i < 6; i++)
+    {
+        dup.m_rtt[i] = m_rtt[i].clone();
+        dup.m_dirty_box[i] = m_dirty_box[i];
+        dup.m_dirty_face[i] = m_dirty_face[i];
+    }
+    return dup;
+}