draw main brush preview first if the queue is long

src/node_stroke_preview.cpp

@@ -509,7 +509,7 @@ void NodeStrokePreview::draw_stroke()
         });
     }
     s_queue.mutex.unlock();
-    s_queue.PostUnique(this);
+    s_queue.PostUnique(this, m_draw_first);
 }
 
 void NodeStrokePreview::draw()