* some cleanup - removed ugly-as-hell exposure of only parts of the
   internal data struct on B2DPolygon (and reworked the only client
   of that code)
 * added stl-style begin/end to B3DPolyPolygon as well, mirroring
   B2DPolyPolygon, plus adapting the insert() func to make use of
   that (avoiding loads of temporaries)