From Marcus Hein,  Added support for OpenGL SSBO and SSBB via osg::ShaderStorageBufferObject and osg::ShaderStorageBufferBinding to core OSG library, and added new osgSSBO example


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14599 16af8721-9629-0410-8352-f15c8da7e697