An option vtkContourFilter::GenerateTriangles is added. This is on by
default--preseving the previous behavior. If turned off, polygons
instead of triangles will be produced.

This can be considered a continuation of what was done for vtkCutter in
commit 8fb71b7f.  Please see that commit for details of the
algorithm.

Change-Id: If8bded0e48ad67bd37af65b35a8b9115e708538a