[vtkusers] Fixing vtkPolyDataToImageStencil

[David Gobbi]

Hi Everyone,

I've created a patch [1] that I hope will fix the "random bad lines" bug [2]
in vtkPolyDataToImageStencil.  In order to test the new code, I'm going
to be using some randomly-generated surfaces, but if anyone out there
is able to test this patch on difficult datasets, your feedback will be very
helpful.

 - David

[1] http://review.source.kitware.com/#/c/17021/
[2] http://www.vtk.org/Bug/view.php?id=14042