In preparation to remove `DistancedHitObjectComposer`, split off
`IPositionSnapProvider` from `IDistanceSnapProvider`.
`DistancedHitObjectComposer` was not touching `IPositionSnapProvider`'s
only interface member at all, it was just forwarding it for subclasses
to override to their own leisure.
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.