window_add_pane
new_wp = window_add_pane(wp->window, NULL, hlimit, 0);
new_wp = window_add_pane(w, NULL, hlimit, sc->flags);
new_wp = window_add_pane(w, sc->wp0, hlimit, sc->flags);
struct window_pane *window_add_pane(struct window *, struct window_pane *,