render() still needs to ask the window to update

otherwise when in a simple qquickview one will have to resize
the window to actually see updates

src/qmlcontrols/kquickcontrolsaddons/plotter.cpp

@@ -709,6 +709,7 @@ void Plotter::render()
 
     // Delete the VBO
     glDeleteBuffers(1, &vbo);
+    window()->update();
 }
 
 QSGNode *Plotter::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *updatePaintNodeData)