Added a template get and get_exisiting method into osg::State that implements a new mechanism for managing OpenGL extensions.

Refactored the BendFunc::Extensions usage to simplify it utilizing the new osg::State extension mechanism.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14560 16af8721-9629-0410-8352-f15c8da7e697