screen: Make sure to initialize NUMBER_OF_DESKTOPS

update_num_workspaces() is a no-op when the number of workspaces
did not actually change. That is fine, except that we still want
to initialize the _NET_NUMBER_OF_DESKTOPS hint on startup to not
break components like nautilus-desktop that rely on it.

https://bugzilla.gnome.org/show_bug.cgi?id=760651

src/core/screen.c

@@ -1215,7 +1215,11 @@ update_num_workspaces (MetaScreen *screen,
   g_assert (new_num > 0);
 
   if (g_list_length (screen->workspaces) == (guint) new_num)
+    {
+      if (screen->display->display_opening)
+        set_number_of_spaces_hint (screen, new_num);
       return;
+    }
 
   last_remaining = NULL;
   extras = NULL;