Polyhedron faces lost when cells are handed to SetCells()

vtkUnstructuredGrid::SetCells(cellTypes, cells) interprets its cell array as a legacy face stream ([nCell0Faces, nFace0Pts, i, j, k, ...]) as soon as any cell is a VTK_POLYHEDRON, and decomposes it. GetCells(), however, returns point lists. A caller that copies cells out of one grid and hands them to another therefore has the point list read back as face descriptions: the first point id is taken as a face count. The result is invalid topology, and on larger meshes very long runtimes inside DecomposeAPolyhedronCell.

Three callers did exactly this, and each now copies the polyhedron faces alongside the connectivity and builds its output through SetPolyhedralCells():

• vtkExtractCellsAlongPolyLine, which backs Plot Over Line. Faces are remapped through the same input to output point map already used for the connectivity.

• vtkOverlappingCellsDetector, which gathers candidate cells with vtkCell::GetPointIds() before exchanging them between ranks. The faces now travel with the candidates and are remapped with them.

• The vtkUnstructuredGrid (de)serialization helper, which wrote only Cells and CellTypes. It now writes PolyhedronFaces and PolyhedronFaceLocations as well. States written without them still load.

Grids containing no polyhedra take the previous code path in every case.

Each fix comes with a test that fails without it. The existing tests could not have caught any of these: they assert on point counts, cell counts and bounds, all of which survive the loss of a polyhedron’s faces.