meta-xwayland: Generate a Xauth file and pass this to Xwayland when starting it
Before this commit, sudo x11-app, e.g. sudo gvim /etc/some-file, fails when running a Wayland session. Where as doing this under a "GNOME on Xorg" session works fine. For a user switching from the Xorg session to the Wayland session, this is regression, which we want to avoid.
This commit fixes this by creating and passing an xauth file to Xwayland when mutter starts it. Just like gdm or startx pass a xauth file to Xorg when they start Xorg.