improve progress bar, refactor node destruction, simplify layers export

src/node_canvas.cpp

@@ -643,9 +643,8 @@ void NodeCanvas::on_tick(float dt)
     m_outline_pan = glm::fract(m_outline_pan + dt * 0.01f);
 }
 
-void NodeCanvas::destroy_immediate()
+void NodeCanvas::destroy()
 {
-    Node::destroy_immediate();
     m_blender_rtt.destroy();
     m_cache_rtt.destroy();
     m_rtt.destroy();
@@ -653,4 +652,5 @@ void NodeCanvas::destroy_immediate()
     m_face_plane.destroy();
     m_line.destroy();
     m_grid.destroy();
+    Node::destroy();
 }