The underlying vtkImageMapToColors contained in the vtkImagePlaneWidget
only considers the active scalar of the input.

The trick is to use a shallow copy of the input image to set the active
scalar on its fields without corrupt other decorators.

Paraview has not such a problem since it does not use the
vtkImagePlaneWidget.

Simplifications have also been made to avoid spurious calls to set the
positions of the slices.