na-xembed: Do not use same coordinates of mutter offscreen windows

We created the socket window using the same coordinates that mutter uses
for its own off-screen windows created via
meta_x11_display_create_offscreen_window() and that are filtered out in
MetaWindowX11's is_our_xwindow().

This caused these windows to be never tracked by mutter and then we
never received a "::window-created" signal that should lead a ShellTrayIcon
to associate itself as the clone of the created MetaWindow.

So, still keep it offscreen, but at different location.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2684>

src/tray/na-xembed.c

@@ -393,7 +393,7 @@ na_xembed_add_window (NaXembed  *xembed,
       priv->socket_window =
         XCreateWindow (xdisplay,
                        meta_x11_display_get_xroot (priv->x11_display),
-                       -100, -100, 1, 1, 0,
+                       -1, -1, 1, 1, 0,
                        priv->xvisual_info->depth,
                        InputOutput,
                        plug_attrs.visual,