Introduce the EGL/X11 winsys, and use it when the OBS_USE_EGL environment
variable is defined. This variable is only temporary, for future commits
will add a proper concept of platform.
All the EGL/X11 code is authored by Ivan Avdeev <me@w23.ru>.
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>
