`SpriteText.TransformSpacingTo` has a considerable overhead

[peppy]

As mentioned in https://github.com/ppy/osu/pull/14510, running a transform on Spacing will cause invalidation of the TextBuilder every frame, leading to considerable allocation overhead. Opening for discussion (we may decide that this is not a cause for concern, but it does stand out as the only framework-supported transform that has this kind of overhead curently).