In VTK 5.10 the vtkImageActor was reimplemented with vtkImageSliceMapper.
As an unwanted side-effect, the use of SetDisplayExtent caused the
GetSliceNumberMin/Max methods to always return the current slice.
This patch fixes the problem and restores backwards compatibility.

Change-Id: If01bfd47cd7bfa3d8ff98a7c8b979f628f23ec23