Moved GL2Extensions functionality into the include/osg/GLExtensions header and new GLExtensions object.
Moved the #defines into new include/osg/GLDefines
Converted all GL2Extensions usage to GLExtensions usage


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14593 16af8721-9629-0410-8352-f15c8da7e697