vtkQuadricDecimation bug fix: more robust against degenerated triangles
This contribution fixes two issues:
- Crash for meshes containing degenerated triangles like [3, 3, 3] (point indices are in parentheses). How it was fixed: avoid triangle destroying twice.
- Early stop decimation if two triangle vertices coordinate coincide. How it was fixed: skip edges with NaN cost (cost for edges sharing such points is NaN).