root@laptop:/# cat /usr/share/xsessions/xgl.desktop 
[Desktop Entry]
Encoding=UTF-8
Name=Xgl
Comment=Start an Xgl Session
Exec=/usr/local/bin/startxgl.sh
Icon=
Type=Application
root@laptop:/# cat /usr/local/bin/startxgl.sh
#!/bin/sh
Xgl :1 -fullscreen -ac -accel xv:pbuffer -accel glx:pbuffer &
DISPLAY=:1
cookie="$(xauth -i nextract - :0 | cut -d ' ' -f 9)"
xauth -i add :1 . "$cookie"
exec dbus-launch --exit-with-session gnome-session