+ avoid creating two coincident vertices (needs more testing)
+ clipper based polygon clipper
  (FIXME: avoid leaking memory)
+ avoid unnecessary const_cast<>
+ test original (un-moded) clipper
+ update clipper to latest (5.1.0)
Merge branch 'master' into clipper

Conflicts:
	include/mapnik/vertex_converters.hpp
+ link clipper obj's into core lib
Merge remote-tracking branch 'origin/master' into clipper
+ revert as it was breaking polygons in some cases
Merge branch 'master' into clipper