wayland: Force decoration update
If the client had created an xdg-toplevel-decoration and has called the set_mode() request with csd before the initial configure event is sent, we still need to send an xdg-toplevel-decoration configure event with csd mode.
!646 (closed) fixes this problem more elegantly, but it's still wip because there are issues with async decoration border updates (decoration state needs to be double-buffered).