Causes several knock-on inspections in `OsuGame` et al. Probably best
addressed in a separate pass, because treatment is mixed at best (some
places nullcheck, some expect non-null).
`ModelBackedDrawable<T>.CreateDrawable()` is R#-annotated to accept
a potentially null model. Apply nullability there too for better reading
experience.
Avoids things like touch screen inputs also being flipped.
Note that these adjustments can't be applied directly to the playfield
due to how playfields are used in various rulesets (basically relying on
the `PlayfieldAdjustContainer` to get things in the right place).
Closes#24000.
This also moves the beatmap skin disable toggle to on toggle, in line
with review feedback.
I've decided to always apply the disable, not just on the `Player`
screen. It should be assumed that if a user is in the skin editor they
are never going to need access to this anyway.