Make electric motors go in reverse better

[wapcaplet]

Summary

SUMMARY: Bugfixes "Give electric motors equal power in both forward and reverse"

Purpose of change

Fixes #38165

Describe the solution

Adds a new vehicle::max_reverse_velocity function as the counterpart to vehicle::max_velocity, checking to see if the vehicle has an active electric motor. If so, the max reverse velocity is equal to max forward velocity. Otherwise, the usual 1/4-power penalty is still applied.

Describe alternatives you've considered

Considered adding a flag to all the electric motors that should be exempt from the reverse-velocity restriction, but a simple check using has_engine_type gives what we need here.

Also considered refactoring the calculations (both with cruise control on and off, since they are handled by separate functions) to be more realistic with respect to low-gear torque as opposed to high-gear maximum speed, but that's a dark hole I dared not descend for this simple bugfix.

Finally, I also considered trying to calculate the power contribution for each motor, in order to more accurately determine a maximum reverse velocity, but this too seemed like overkill for solving this problem.

Testing

Debug-spawned some vehicles, tried different gasoline and electric motors, and loaded them with large storage batteries for weight until they couldn't move any more.

Electric car with small electric motor (11 lbs, Power+7040)

• 15,447 lbs: forward/reverse OK
• 15,888 lbs: too heavy for its engines!

Full disclosure, I had similar results with a gasoline engine - in other words, I couldn't reproduce the original bug with a gasoline-powered engine. But there, the engine power and eventual maximum weight were in the same ballpark:

Beetle with 0.4L 1-cylinder gasoline engine (44.1 lbs, Power +7370)

• 12,526 lbs: forward/reverse OK
• 12,967 lbs: too heavy for its engines!

In other words, it is difficult to reach that edge case where the vehicle is just barely too heavy to move in reverse. Typical return values from max_velocity for an unloaded vehicle are in the thousands, so I think one would need to find that narrow range 0 < max_velocity < 4 to encounter the bug.

Newly-added unit tests now verify the existing 25% reverse velocity for a combustion engine, and 100% reverse velocity for electric engines.

Additional context

Reverse velocity calculation is still extremely crude; as I noted in #38165, before this PR, all vehicle engines were simply nerfed to 1/4 velocity while in reverse, regardless of whether it's powered by pedals, electric motors, gas engines, or a horse. This seems to have an effect on both torque (accelerating from 0 on various terrain) and top speed. But the game does not yet model gearboxes or drive wheels or hybrid-electric drivetrains (or does it?), so this mathematical shortcut gives a similar result in most cases - that most vehicles can't go in reverse very quickly.

But electric motors have equal torque in either direction, so vehicles powered by electric motors should certainly be exempt from the 1/4-power nerf in the reverse direction. In theory, they can do just as well in reverse (leaving aside any questions of how difficult it might be to actually drive at high speed while looking and steering backwards - better level up that driving skill!)

This PR had some scope creep - now the majority of changes are related to unit tests and test vehicle data, and only a few lines relate to the original bugfix.

[SirPendrak]

What if vehicle has two engines, one electric and one standard, all active and then trying to go reverse?

[wapcaplet]

What if vehicle has two engines, one electric and one standard, all active and then trying to go reverse?

@SirPendrak As noted in my commit message for ddd9370, the presence of any electric motor in the drivetrain (no matter how small) will negate the reverse-power penalty. This PR does not calculate the power contribution for each motor.