[vtkusers] Set position of vtkImageSlice

[David Lonie]

Hi list,

I'm writing a test for some new functionality that will generate
several vtkImageData instances and render them at different positions.

I'm setting up a vtkImageSlice like this:

  vtkNew<vtkImageData> image1;


  vtkNew<vtkImageSliceMapper> mapper1;
  mapper1->SetInputData(image1.GetPointer());
  vtkNew<vtkImageSlice> actor1;
  actor1->SetMapper(mapper1.GetPointer());
  actor1->SetPosition(10, 10, 10);