[vtkusers] Can't transform a vtkVolume

[Lorenzo]

Hello everyone

I'm having a strange problem related to VTK volume rendering: I can't apply
a simple transform (a translation) to volumes. I'm using the
"SetUserTransform()" function that vtkVolume inherit from vtkProp3D, a
method that works like a charm on vtkActors. I also tried "SetUserMatrix()"
and simple "SetPosition()" but nothing.

Here's my code: http://pastebin.com/7MnTCHHN
And here's the output: http://i.imgur.com/feCyaIL.png

As you can see, I generated two cube volumes (one red and one blue) and two
cone actors (one red and one blue). I then apply the same transform on both
blue props but only the cone actually translates (that's why you see a
purple volume, the two cubes are in the same position).

How is this possible?