+ 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
add the ability to set the cached envelope of a memory_datasource
Merge branch 'master' of github.com:mapnik/mapnik
Merge remote-tracking branch 'origin/master' into clipper
+ revert as it was breaking polygons in some cases
make M_PI visible to msvc
remove uneeded default args to projection ctor (also fixes msvc linking)
Merge pull request #1722 from mapnik/projection-ctor-fix

Projection ctor fix
move rule cache off style objects as styles should be immutable - amends 1c4b67ba98b12ace - refs #916
remove uneeded rule_cache property
also clean up pyc files - helps avoid debian packaging issues
Merge branch 'master' of github.com:mapnik/mapnik
Merge branch 'master' of github.com:mapnik/mapnik into fix-rule-cache
add support for background-image to cairo_renderer and refactor into setup function to match agg/grid renderers - closes #1724