[vtkusers] Added Quad-Support for Subdivision Filter

[Scheucher, Lukas]

Hello,

I have had a problem with the visualization of quad-cells, as the
representation of bilinear solutions as two triangles created unwanted
artifacts. I hoped, that as a workaround, I could apply the subdivision
filter until a sufficiently smooth representation is achieved.

Unfortunately the subdivison filter demands triangles as input, which
was not a solution since triangulating manifests the problem.

I therefore modified the subdivision filter
(vtkLinearSubdivisionFilter.cxx, vtkInterpolatingSubdivisionFilter.cxx)
so that it is able to subdivide not only triangle-, but also quad-cells.

The filter now subdivides each cell into multiple cells of its own type
using (bi)linear interpolation. Unsupported cells continue to be skipped.

I would appreciate any feedback, whether this feature is wanted and if
it might make into VTK and also which further changes are necessary.


Attached to this mail you will find my patch.


Regards,

Lukas Scheucher
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