Fix for polyhedron cells.

This happens for datasets containing a mixture of cell types.  The
vtkIdList was not reset consistently between polyhedron cells and other
cell types.

Change-Id: I482d9225f5dac305ba742287145dc53dd51f31c3
Merge topic 'fix-append-filter-for-polyhedron-cells' into master

ac52a402 Fix for polyhedron cells.