cherry picking..
+ filter_at_point - cache inflated bbox
+ use tol in feature_at_point
msvc on windows breaks with singleton declared but gcc needs default visibility to avoid double singleton registration across dlls when using -fvisiblity=hidden

Conflicts:
	include/mapnik/utils.hpp
+ mapnik_datasource parameters : handle unicode objects - convert to internal UTF8 representation
+ utf8 to/from utf16 implementation

Conflicts:
	include/mapnik/utils.hpp
+ add accidentally removed config.hpp