Brought the UIStack over from the previous game to use here.

src/guecs/ui.cpp

@@ -118,6 +118,8 @@ namespace guecs {
     }
   }
 
+  // BUG: either render detects that the things are initialized or there's
+  // another validator function I can call in debug modes to confirm they are
   void UI::render(sf::RenderTarget& window) {
     if(auto bg = get_if<Background>(MAIN)) {
       bg->render(window);