Move the GLX-related code to gl-x11-glx, and introduce gl-nix as
a winsys-agnostic abstraction layer. gl-nix serves as the runtime
selector of which winsys going to be used. Only the X11/GLX winsys
is available now, but later commits will introduce the X11/EGL
winsys as well.
The gl-nix code was originally written by Jason Francis <cycl0ps@tuta.io>
This is in preparation for the future abstraction layer (gl-x11-*)
and also to match the actual name of the windowing system. When
running under X11, we can glue OpenGL through GLX or EGL, so the
new file name matches that now.
