- There will be triangles rendered inside the mesh (when
    surface-rendering), because one of the cell's triangles is defined
    as a quadrangle in VTK_WEDGE.

  - Therefore, this VTK_WEDGE representation is only used when
    decomposing the mesh, otherwise the correct representation is done
    by VTK_POLYHEDRON.

  - Furthermore, using VTK_PYRAMID gave worse result, because it renders
    2 triangles inside the mesh for the collapsed quadrangle, likely due
    to mismatch with the adjacent cell's face.

  - Using VTK_HEXAHEDRON was not tested in this iteration, given that it
    should give even worse results, when compared to using VTK_PYRAMID.

Patch contributed by Bruno Santos
Resolves bug-report http://bugs.openfoam.org/view.php?id=2099