xwayland: "Accept" confineTo on InputOnly windows

Of sorts, actually make it confine to the pointer focus, as the
InputOnly window is entirely invisible to xwayland accounting,
we don't have a xwl_window for it.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit fafdb0cc)