Extent on its output when input is NULL.  This makes life easier for filters
further down the line.
ERR: vtkImageEuclideanDistance now checks whether the inPtr is NULL before
blindly writing to it.
ERR: vtkImageIterateFilter checks that the input is non-NULL before
updating data.

`

clip 2D cells as well.  It is not absolutely essential that these
changes make it into the release, because D3 only needs to clip
cells for volume rendering, and we don't have volume rendering yet.