[vtkusers] updating vtkActor

[scottjp at CLEMSON.EDU]

I'm trying to provide some loose "animation" for a VTK display of mine. I
just need to move a vtkVertex upon a timer. I have:
- a vtkPolyData instance with the point and cell
- a vtkPolyDataMapper
- a vtkActor.

When I change the point's coordinates, I call Update on the
vtkPolyDataMapper - updating the pipeline - and I call Render. However,
this does *not* update the vtkActor. In order to get this to work
properly, I have to call removeActor and addActor. Why? How can I update
an actor without removing and adding it again?

Thanks,
Scott