[vtkusers] Help about refreshing of vtk window

[David Doria]

> I mean I use the following codes to initialize these variables again.
>
> m_ren = vtkSmartPointer<vtkRenderer>::New();
> m_renWin = vtkSmartPointer<vtkRenderWindow>::New();
> m_iren = vtkSmartPointer<vtkRenderWindowInteractor>::New();

You should probably not be recreating the variables. Instead just
change the content of the renderer and then call Render().

David