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.