For some reason, spinner judgements would stack from the top left in an awkward fashion. This patch ensures that the judgement location will always be in the centre of the screen.
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.
This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.