Calling Expire before LoadComplete is unsafe

[peppy]

Right now, calling Expire before a clock has arrived at a drawable can lead to incorrect lifetimes (due to a lookup on LatestTransformEndTime -> TransformStartTime):

osu-framework/osu.Framework/Graphics/Transforms/Transformable.cs

Line 35 in 496219e

public double TransformStartTime => (Clock?.CurrentTime ?? 0) + TransformDelay;

If possible, we should remove this requirement to make it a safe call, else it should probably throw an exception.