[vtkusers] Changing the orientation of the Terrain interactor style

[Jordi Gutiérrez Hermoso]

I want to visualise surfaces of the the form z=f(x,y), so the Terrain
interactor seems to be the best suited for it. However, how can I
specify what the "top" of the surface is? Right now the side ends up
being the top, so it doesn't work as you would expect.

Thanks,
- Jordi G. H.