Moved local Extensions structs into GL2Extensions


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14584 16af8721-9629-0410-8352-f15c8da7e697
Removed no longer used Extension definition


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14585 16af8721-9629-0410-8352-f15c8da7e697