Slider heads are guaranteed to always be drawn at (0,0). This fixes
weird behaviour in the editor, but also simplifies things in the
process. Win-win.
Closes#20644.
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.