Move title usages of towerbg to titlebg

With the previous commit in place, we can now simply move some usages of
the previous towerbg to use a separate object instead. That way, we
don't have to mess with a monolithic state, or some better way to phrase
what I just said, and we instead have two separate objects that can
coexist side-by-side.

desktop_version/src/Script.cpp

@@ -2642,7 +2642,7 @@ void scriptclass::resetgametomenu()
 	graphics.flipmode = false;
 	obj.entities.clear();
 	graphics.fademode = 4;
-	graphics.towerbg.tdrawback = true;
+	graphics.titlebg.tdrawback = true;
 	game.createmenu(Menu::gameover);
 }