additional catchup mechanic by lowering maxspeed

[complyue]

the catchup mechanic in master branch has a disadvantage that it only leans toward player against AI. actually, when the player leads a lot in the race, he/she may possibly want the AIs to catchup too, so as to fight more with them for more fun.

this PR experiments with the idea to lower the maxSpeed limit of a car one before another in order.

added GP param catchupFactor = 20, which would effectively grant the behinder 1x advantage wrt maxSpeed, for every 1/20 distance of a single lap, that between the 2 cars.

turboStrength = 200 is found so large that it'll overwhelm the catchup effect added, so lowered to 70 initially for experimenting purpose.

please try & see how you feel about the effects so far?

[complyue]

#438 is a later attempt, which I feel more effective after played a while.

[agateau]

I can see how this can make races more challenging for good players, but this change has the side-effect of making the 1st vehicle slower. This is a problem for players who try to beat their record times. What do you think?

[complyue]

this is not that effective as #438, let's focus discussion about this idea there.