Union and symmetric difference produce same result.

The intersection is not discarded using symmetric difference in the intersect app.

I think that SymmetricDifferenceCollector::collect should not have the following code:

```
else if (face_class == FACE_IN) {
       REV(orig_face, vertices, normal, poly_a, face_class, hooks);
 }
```

When I comment these lines out, I get the expected results.
