Update VolumePicker test to use vtkImageSliceMapper
The vtkImageActor previously used in this test was not compositing correctly with the volume mapper, it seems that it was not drawing into the depth buffer. Replacing it with vtkImageSliceMapper fixes the compositing.
See #19368 (closed) for further discussion of the difference in compositing between vtkImageActor and vtkImageSlice.
Edited by David Gobbi