This commit acclerates geometry extraction of vtkGeometryFilter by
1) tuning data access; 2) adding a new, higher-performing method to
vtkUnstructuredGrid::IsCellBoundary(); and 3) threading the algorithm.
At this point typical performance improvements are 5-10x over
vtkDataSetSurfaceFilter (which is harder to thread because the global
face hash table is a parallel bottleneck, and requires
additional support for compositing the hash table.)