`WorldStep()` does not work correctly when simulating multiple worlds

[mikuso]

The WorldStep helper function appears to use a variable _accumulator to store the amount of "unused" time after performing a variable number of timesteps. However, this accumulated time value is shared across all worlds being simulated.

It seems to me that if I were to pass a different worldId via the data parameter, then it should have its own accumulator independent from another.

Hope this makes sense.