+ 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
move rule cache off style objects as styles should be immutable - amends 1c4b67ba98b12ace - refs #916
remove uneeded rule_cache property
Merge branch 'master' of github.com:mapnik/mapnik into fix-rule-cache
Merge branch 'master' of github.com:mapnik/mapnik into fix-rule-cache
Merge branch 'master' into clipper
+ no need to fwd decl mapnik::Map
+ update viewer
Merge pull request #1723 from mapnik/fix-rule-cache

Fix rule cache (TODO: use move semantics for rule_cache)
+ use boost::move to emulate 'move' semantics
  for mapnik::rule_cache objects and avoid
  dynamically allocating them.
sync with master