Fix osu!mania beat snap grid sometimes not displaying correctly

[peppy]

I'm not sure what the cause of the issue is, but I'm also not sure why it wasn't using DrawablePool (was it not around back then?).

Either way, switching to a proper pool fixes things just fine.

Resolves #25009.

[bdach]

The cause of the issue is lifetimes not being properly reset on DHO reuse. While this line:

osu/osu.Game.Rulesets.Mania/Edit/ManiaBeatSnapGrid.cs

Line 137 in b3d3ae8

line.HitObject.StartTime = time;

would correctly update LifetimeStart based on HitObjectEntryManager logic, the following logic was not re-ran:

osu/osu.Game.Rulesets.Mania/Edit/ManiaBeatSnapGrid.cs

Lines 197 to 201 in b3d3ae8

	protected override void UpdateInitialTransforms()
	{
	// don't perform any fading – we are handling that ourselves.
	LifetimeEnd = HitObject.StartTime + visible_range;
	}

causing reused objects to keep stale LifetimeEnd and as such disappearing into the void when scrolled far enough.

In that light, this fix is fine, because hitobject application calls onDefaultsApplied(), which calls updateState(), which calls UpdateInitialTransforms().