Mark DrawNode::clear as virtual function

Author: halx99

core/2d/DrawNode.h

@@ -521,7 +521,7 @@ class AX_DLL DrawNode : public Node
                            float thickness = 1.0f);
 
     /** Clear the geometry in the node's buffer. */
-    void clear();
+    virtual void clear();
     /** Get the color mixed mode.
      * @lua NA
      */