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 5c44c134)