wayland: Create XAuthority file once

Where the prepare_auth_file() call is, it does create a new one on every
respawn of Xwayland. This is not benefitial, as the XAUTHORITY envvar is
already fixed in the session.

Only create the XAuthority file once, and reuse it on future Xwayland
respawns.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/728

src/wayland/meta-xwayland.c

@@ -743,6 +743,9 @@ meta_xwayland_init (MetaXWaylandManager *manager,
     {
       if (!choose_xdisplay (manager))
         return FALSE;
+
+      if (!prepare_auth_file (manager))
+        return FALSE;
     }
   else
     {
@@ -750,9 +753,6 @@ meta_xwayland_init (MetaXWaylandManager *manager,
         return FALSE;
     }
 
-  if (!prepare_auth_file (manager))
-    return FALSE;
-
   manager->wayland_display = wl_display;
   policy = meta_get_x11_display_policy ();