BUG: Fix #122 black analytic objects

Fix regression introduced in !121 (merged) with the use of the fast normals filter vtkTriangleMeshPointNormals. That filter was also applied for surface meshes with non-triangular cells: cells, sphere, capsule, cube (quads and not triangles).

Since the issue of non-triangular meshes comes from analytic mesh sources, the output surface meshes will always be rigid, so their normals will stay constant and won't need to be recomputed. We can, therefore, use the filter vtkPolyDataNormals, which is slower but will only need to be called once if no deformation occurs.