glx: Flush xlib state before calling xcb_glx_is_direct

This is an awful fix and I hate it, but it fixes glXCreateNewContext sporadically failing with glmark2 and a kopper-enabled zink. The reason I hate it is the error generated really is coming from the server, because the xlib-style GLXCreateNewContext we just tried to emit hadn't been flushed yet, so when the GLX server goes to look up the context it indeed is not created yet. And if we have that kind of problem with handling the xlib/xcb handoff, surely this is not the only place.