xwayland: fix xdg->xwayland drag-and-drop

As struct wlr_drag is destroyed on drop and in the process resets
the focus, a xwayland dnd listener would also reset xwm->drag_focus.

This prevents the xcb replies from being processed and also prevents
the transfer if a compositor would not additionally request new focus
in its wlr_drag destroy handler (which is something usually only done
when in a focus-follows-mouse setting).

This patch creates a new xwm->drop_focus pointer which is a copy of
xwm->drag_focus at drop time. The xcb reply handler and transfer
logic now use the new xwm->drop_focus for their authorization checks.

(cherry picked from commit 546c5d000d)

xwayland/xwm.c

@@ -2177,6 +2177,7 @@ struct wlr_xwm *xwm_create(struct wlr_xwayland *xwayland, int wm_fd) {
 	wl_list_init(&xwm->pending_startup_ids);
 	wl_list_init(&xwm->seat_drag_source_destroy.link);
 	wl_list_init(&xwm->drag_focus_destroy.link);
+	wl_list_init(&xwm->drop_focus_destroy.link);
 
 	xwm->ping_timeout = 10000;