They had scale transforms applied to them in two places: the actual
legacy pieces themselves (esp. `LegacyHitCirclePiece`), and on the
`DrawableSliderRepeat` level.
This change moves all of the scale transforms to the skinnable pieces.
Argon and triangles have received a copy of the previous logic each,
so behaviour on those skins should not change.
Aims to make minimal changes to `DrawableSlider` itself. I'm not super
happy about the slider ball being moved above the head circle, but it
*is* what people are used to so no one except for me is going to
complain.
Supersedes and closes https://github.com/ppy/osu/pull/14561.