apple: Implement applegl_unbind_context

glXMakeCurrent(dpy, None, NULL) would not correctly unbind the context
causing subsequent GLX requests to fail in peculiar ways

http://xquartz.macosforge.org/trac/ticket/514

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
(cherry picked from commit 5c44c1348ea13f51a1616968daa7034bb48e42b1)