ENH: Add SetOutputSpacing/SetOutputOrigin methods to vtkImplicitFunctionToImageStencil and vtkPolyDataToImageStencil.  In VTK 4, the Spacing and Origin used to be taken from the consumer of the vtkImageStencilData via an ugly hack which does not work in VTK 5.  In VTK 5, the Spacing and Origin must be set explicitly.