[vtkusers] vtkExtractEdges functionality

[David Doria]

I recently discovered that if you run vtkExtractEdges twice, you are
left with nothing. Would it make sense for vtkExtractEdges to pass
vtkLines through to the output? In my case, I have a filter that takes
a polydata, and I'm not sure if the edges have already been extracted.
I though tit would make sense to run vtkExtractEdges at the beginning
of the filter to make sure there were only edges, but if the input
already consists of only edges, the result of this second pass is an
empty polydata.

Any opinions on this? Or cases that it wouldn't make sense to pass
along vtkLines?

Thanks,

David