Fuel fuel_top_speed_multiplier to affect 'carprototype'

[Fishbus]

Hey,

I did some messing around with fuels a long time ago with fuel_top_speed_multiplier & fuel_acceleration_multiplier and this quirk still remains.

I'd expect that both car/locomotive prototypes would behave similarly in regards to fuel modifiers, but they work slightly differently:

Cars

• fuel_acceleration_multiplier -> works | affects acceleration & top speed.
  fuel_top_speed_multiplier -> no effect.

Locomotives

• fuel_acceleration_multiplier -> works - affects acceleration only.
  fuel_top_speed_multiplier -> works - affects speed only.

I presume the car and train acceleration and speed models are different enough it makes it tricky, since there are different power/weight properties

Cheers

[Rseding91]

I implemented this for 2.1.