Things were a bit jumpy when a screen was adjusting `ParallaxAmount`. This smoothes the applied scale changes to look great again. Most noticeable when hitting the retry hotkey (`~`) from gameplay.
This also cleans up some shared code in OsuScreen.
They have huge overheads. See ppy/osu-framework#1411.
Uses GetContainingInputManager instead, as per framework changes.
Also accounted for the ParallaxAmount when moving to default position
Kinda hacky but will do for now.