[vtkusers] set vtkInteractorStyleTrackballCamera rotation center

[Florian Bruckner]

hi,

is it somehow possible to set the rotation center when using
vtkInteractorStyleTrackballCamera(). My problem is that if i move my
geometry out of the center of the renderer, the rotation is still around
the center of the renderer and not around the center of the geometry.
Using SetFocalPoint() allows to set the focalpoint of the camera to the
geometry, but then the geometry is centered again. 

in fact i want the have the same behaviour the i get with
vtkInteractorStyleTrackballCamera() for one geometry, but it should also
work for many geometries, without having to synchronize to motion of
many actors. In my opinion moving the camera would be the best way to do
this, but i did not archive this.

thanks
FloB