Don't calculate safe speed for stationary vehicles for SFX

[prharvey]

Summary

Performance "Optimize stationary vehicle performance when they contain a lot of items."

Purpose of change

Turning the engine on generates SFX, and that requires getting the safe speed of a vehicle, which requires getting the mass of all items on the vehicle. This isn't needed if the vehicle is stationary.

Fix #68982

Describe the solution

Delay computing the safe speed until it is needed.

Describe alternatives you've considered

This could be fixed for moving vehicles by caching the item mass, but that would require a lot of investigation into where that cache would need to be invalidated. It's probably fine for moving vehicles to slow the game down, since that is done interactively most of the time and won't be noticed.

Testing

Additional context

Before:

After: