wip: use the glx-to-egl adaptor outside of just Xwayland

Open Adam Jackson requested to merge ajax/xserver:dix-glxegl into master

We're still using magic dlsyms into the DRI driver to set up GLX, even though when we're using glamor there's a perfectly good window system and GL we could set this up from instead. Possible wins here:

  • modesetting driver using egl adaptor
  • Xephyr ported to EGL and using adaptor
  • EGL adaptor taught how to do indirect GLX
  • Factor out enough from EGL adaptor to be usable without glamor

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