vtkGeometryFilter: incorrect values with image data + blanking

I've add a new test TestDataSetSurfaceFilterWithBlankedImageData. When using vtkGeometryFilter to extract the surface, the result is totally wrong. See the attached image. The top two images are computed using vtkDataSetSurfaceFilter, the lower two uses vtkGeometryFilter. The results should have been identical to the one on the top.

One this is fixed, please update the baseline as well.