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.