[vtkusers] cutting through unstructured grid

[Philippe Guerville]

>   The problem is that most renderers (including OpenGL) do not handle
non-planar polygons consistently.

I fully agree. Furthermore, display is faster with triangles.

But I've got the same special need as Simon has. I would like to cut through a
hexahedron mesh and 90% of the time it is using a plane (so with planar polygons
handled correctly by OpenGL). When you see triangles, it's hard to understand
the relation with mesh. So, it is why I've done this modification in my
application to preserves the outlines of the original cells.

As Simon suggested to me, we can rename Contour method using polygons
PolyContour and had a flag to vtkCutter to switch between contouring methods
(polygons and triangles) depending on what you needed to do?
And so, vtkContour filter or other filters will not be affected.

    Philippe