replace zero-sum offsets with union_ex()

The negative offset exposes a Clipper quirk that rarely reverses
winding order, which clobbers holes. 

After applying commit in #720 (increase scale factor for offset), this model, with this config, will lose holes on the first layer.

http://dl.dropbox.com/u/315849/Slic3r/z-motor-mount.stl
http://dl.dropbox.com/u/315849/Slic3r/layerfill/config-filltest.ini

(#720 doesn't cause this, it just happens to help set up the conditions that cause this Clipper quirk.)

It's a rare thing, but I was able to narrow it down to Clipper (v4.8.4), and a specific test polygon that I extracted from the test case. An earlier Clipper didn't produce the same error. And a very small change in one coordinate would make the problem go away. I've had similar experiences with winding reversals with Clipper negative offsets in a different project.

But the issue is avoided here if we skip the outset-inset cycle. It looks like that strategy developed to reduce some of the issues that are addressed by #720.

Maybe there's another purpose it serves that I can't see. If not, reducing that to just a union_ex() fixes this test case, and eliminates the risk of losing holes randomly in general.