Move the OBS_USE_EGL environment variable check to obs-app.cpp,
and set the OBS platform to be either OBS_NIX_PLATFORM_X11_GLX
or OBS_NIX_PLATFORM_X11_EGL.
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>