Fix looking up GLX functions with glXGetProcAddress before a display is available

Created by: kbrenneman

Calling glXGetProcAddress is supposed to be able to look up a function independent of the display. But, if libglvnd hasn't loaded a vendor library yet, then it won't be able to find a dispatch function for any GLX functions.

Worse, if the app asks for a GLX function that no vendor library provides, then it will generate and return a GL dispatch function.