Last I checked (which was years ago, admittedly) the GLX libraries still required Xlib. There were XCB functions for GLX, but they didn't work with the client-s

Last I checked (which was years ago, admittedly) the GLX libraries still required Xlib. There were XCB functions for GLX, but they didn't work with the client-side libraries, so you had to use Xlib. Of course, you should just skip GLX nowadays and use EGL instead, which I'm pretty sure does work with XCB.