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>
