[vtkusers] Rendering Multiple Volumes

[Nigel.Hoschke at csiro.au]

At the moment I have two sets of structured points data that are read from
.vtk files using the vtkStructuredPointsReader.  These are both passed
through vtkVolumeTextureMapper2D into separate vtkLODProp3D which are then
added to the vtkRenderer using the AddVolume method.  The outputs of the
vtkVolumeTextureMapper2D are added to the vtkLODProp3D with the ADDLod
method that takes in a vtkVolumeProperty.  A user defined
vtkPiecewiseFunction has been used with SetScalarOpacity method for the
vtkVolumeProperty to make most of the volumes translucent.