Modify Stencil Iterator to work under block mode.

The implementation assumes that SliceEndIncrement will return offset to return a pointer from the slice end to a new slice's start. Continuous Increment doesn't return that. Instead, it assumes that your pointer will be at the beginning of the last row. This is how it is done in vtkImageIterator, which firsts increment to the next row and then adds the SliceEndIncrement offset.