These were being serialized as the base type. On deserialization, due to
the HitWindow of objects being non-null, they would not get correctly
initialised by the CreateHitWindows() virtual method.
- Closes#10403
This matches osu-stable. When the track is running, seeking backwards
(against the flow) is harder than seeking forwards. Adding a mutliplier
makes it feel much better.
Note that this is additive not multiplicative because for larger seeks
the (where `amount` > 1) we don't want to jump an insanely huge amount -
just offset the seek slightly to account for playing audio.