Fix slider tails sometimes not dimming correctly

Originally noticed during review of another change:
ppy#27369 (comment).

`DrawableOsuHitObject` tries to solve the initial dimming of objects
by applying transform to a list of dimmable parts. For plain drawables
this is safe, but if one of the parts is a DHO, it is not safe,
because drawable transforms can be cleared at will.

In particular, on first use of a drawable slider,
`UpdateInitialTransforms()` would fire via `LoadComplete()` on the
`DrawableSlider`, but *then*, also via `LoadComplete()`,
the `DrawableSliderTail` would update its own state and by doing so
inadvertently clear the dim transform just added by the slider.

To fix, ensure dim transforms are applied to DHOs
via `ApplyCustomUpdateState`.

osu.Game.Rulesets.Osu/Objects/Drawables/DrawableOsuHitObject.cs

@@ -80,6 +80,17 @@ protected override void UpdateInitialTransforms()
             base.UpdateInitialTransforms();
 
             foreach (var piece in DimmablePieces)
+            {
+                // if the specified dimmable piece is a DHO, it is generally not safe to tack transforms onto it directly
+                // as they may be cleared via the `updateState()` DHO flow,
+                // so use `ApplyCustomUpdateState` instead. which does not have this pitfall.
+                if (piece is DrawableHitObject drawableObjectPiece)
+                    drawableObjectPiece.ApplyCustomUpdateState += (dho, _) => applyDim(dho);
+                else
+                    applyDim(piece);
+            }
+
+            void applyDim(Drawable piece)
             {
                 piece.FadeColour(new Color4(195, 195, 195, 255));
                 using (piece.BeginDelayedSequence(InitialLifetimeOffset - OsuHitWindows.MISS_WINDOW))