Fix the problem of changing field in slice decorator.
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.
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