ENH: Add new image mappers to VTK.
New class vtkImageSliceMapper will draw slices of a 3D image, while new class vtkImageResliceMapper will draw oblique reformatted slices. New class vtkImageSlice is the corresponding Prop3D for the mappers, and is a replacement for vtkImageActor, which is now a subclass of it. New class vtkImageProperty controls window/level, lookup table, etc. Change-Id: Id6816402404c8d18d1523dec2c36bd809a060170