vtkReflectionFilter: fix cell point ids in reflected triangle strips

With CopyInput on, triangle strips with an even number of triangles were not reflected correctly. The first four cell ids were not set correctly because the number of points in the copied input was not added to these point ids, so they pointed to points in the copied input, not the reflected points. Fix that by computing a point id offset that depends on the CopyInput property and applying it to all cell ids when reflecting triangle strips that match this criteria.