Add missing load delay

That was a bad one... `ModelBackedDrawable` has a default of 0ms load delay, while previously the wrapper created for beatmap set cover used default 500ms, this change is bringing the load delay back, to avoid firing hundreds of web requests just when doing a quick long scroll on beatmap listing.

osu.Game/Beatmaps/Drawables/UpdateableBeatmapSetCover.cs

@@ -36,7 +36,9 @@ namespace osu.Game.Beatmaps.Drawables
             };
         }
 
-        protected override double TransformDuration => 400.0;
+        protected override double LoadDelay => 500;
+
+        protected override double TransformDuration => 400;
 
         protected override DelayedLoadWrapper CreateDelayedLoadWrapper(Func<Drawable> createContentFunc, double timeBeforeLoad)
             => new DelayedLoadUnloadWrapper(createContentFunc, timeBeforeLoad);