DMA-BUF is a widespread Linux buffer sharing mechanism. It is what's
commonly used zero-copy screen sharing by Wayland compositors.
Add a new 'device_texture_create_from_dmabuf' vfunc to gs_exports,
and stub implementations to libobs-opengl. Add a new public method
gs_texture_create_from_dmabuf() that calls this vfunc.
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.
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>
