Reworked algorithm for Voronoi Offset curve extraction.

Now the algorithm is very different from the OpenVoronoi implementation
and hopefully it is now correct (save numerical issues, which will be
a big PITA).

src/libslic3r/Line.cpp

@@ -135,4 +135,4 @@ BoundingBox get_extents(const Lines &lines)
     return bbox;
 }
 
-}
+} // namespace Slic3r