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

pkgs/development/libraries/mesa/llvmpipe-hook.sh

@@ -1,12 +1,26 @@
 # 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