Fix stutter on intensive storyboards when entering break time with 100% background dim

[peppy]

This avoids piled-up overhead when entering break time. It's not great, but it is what we need for now to avoid weirdness.

Addresses #31465.

[bdach]

I got this freak crash stack trace in testing this when messing with the storyboard toggle during player load:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at osu.Game.Storyboards.Drawables.DrawableStoryboard.get_DrawScale() in /home/dachb/Documents/opensource/osu/osu.Game/Storyboards/Drawables/DrawableStoryboard.cs:line 38
   at osu.Framework.Graphics.Drawable.get_IsPresent()
   at osu.Framework.Graphics.Drawable.set_AlwaysPresent(Boolean value)
   at osu.Game.Screens.Play.DimmableStoryboard.<LoadComplete>b__11_0(ValueChangedEvent`1 show) in /home/dachb/Documents/opensource/osu/osu.Game/Screens/Play/DimmableStoryboard.cs:line 85
   at osu.Game.Screens.Play.DimmableStoryboard.LoadComplete() in /home/dachb/Documents/opensource/osu/osu.Game/Screens/Play/DimmableStoryboard.cs:line 72

Feels borderline out of scope, but maybe worth adding some safeties here and there to prevent this from being hit...

[bdach]

see above

[peppy]

That's painful. I've applied band-aid..

[bdach]

it's a stop-gap, but it'll do