Add missing destroy calls to server_fini

Rootston calls "wlr_xwayland_destroy" and "wl_display_destroy_clients"
on shutdown, but these were not called by Sway. Without them, Sway
crashes on exit before the display destroy event handler could be
called. This causes two problems:

 - The TTY is not reset, and it locks up after exiting Sway.
 - drmDropMaster is not called, and the implicit drop (that should
   occur when the DRM fd is closed) seems not to be working in some
   scenarios (e.g. if you have a tmux session running - maybe the fd
   is retained somehow by tmux?). In other words, it you exit Sway,
   you can't start it (or any other program that wants to be DRM
   master) again until you close all your tmux sessions.

sway/server.c

@@ -142,6 +142,10 @@ bool server_init(struct sway_server *server) {
 
 void server_fini(struct sway_server *server) {
 	// TODO: free sway-specific resources
+#ifdef HAVE_XWAYLAND
+	wlr_xwayland_destroy(server->xwayland.wlr_xwayland);
+#endif
+	wl_display_destroy_clients(server->wl_display);
 	wl_display_destroy(server->wl_display);
 	list_free(server->dirty_containers);
 	list_free(server->transactions);