mesa.llvmpipeHook: fix GLX, add GBM, add comments

# shellcheck shell=bash

# Mesa: force software rendering

# https://docs.mesa3d.org/envvars.html

export LIBGL_ALWAYS_SOFTWARE=true

export LIBGL_DRIVERS_PATH=@mesa@/lib/dri

# GLX

# glvnd just does dlopen("libGLX_%s.so"), so we have to resort to

# LD_LIBRARY_PATH, which is bad but what can you do.

# FIXME: maybe change glvnd to allow specifying this properly

export LD_LIBRARY_PATH=@mesa@/lib:${LD_LIBRARY_PATH:-}

export __GLX_VENDOR_LIBRARY_NAME=mesa

# EGL

# https://github.com/NVIDIA/libglvnd/blob/master/src/EGL/icd_enumeration.md

export __EGL_VENDOR_LIBRARY_FILENAMES=@mesa@/share/glvnd/egl_vendor.d/50_mesa.json

# GBM

export GBM_BACKENDS_PATH=@mesa@/lib/gbm

export GBM_BACKEND=dri

# Vulkan

# https://github.com/KhronosGroup/Vulkan-Loader/blob/main/docs/LoaderInterfaceArchitecture.md

# glob because the filenames contain an architecture suffix

# echo is needed to force-expand the glob