Improve vtkUnstructuredGridGeometryFilter performance
MR !3251 (merged) and !3426 (merged) introduced a huge performance regression when using vtkUnstructuredGridGeometryFilter. This is noticeable with a big dataset as I explained in #17146 (closed).
This has been fixed by allocating surface element points on the heap instead of using a fixed size array of 128 points. With this change, vtkUnstructuredGridGeometryFilter performance is almost similar to what it was before. The small loss when compared to VTK 7 comes from heap memory allocation, however this remains quite negligible compared to the gain.
Fix #17146 (closed).