make lwjgl GL* classes non-final

This patch makes makes the LwjglGL* classes public, this is consistent with the other GL* implementations. In my application I use the 'same' GL interface as libgdx but libgdx is only possible one backend. To add my interface the Implementation needs to be non-final:


        // wrap LwjglGL20 to add GL20 interface
	static class GdxGL extends LwjglGL20 implements org.oscim.backend.GL20 {
		@Override
		public void glGetShaderSource(int shader, int bufsize, Buffer length, String source) {
			throw new IllegalArgumentException("not implemented");
		}
	}