Implement max steering constraint

[netgusto]

As explained here: ByteArena/ba#15 (comment)

Pseudocode of current implementation (incorrect):

In pseudocode:

var curvelocity Vec2 = GetCurrentVelocity()
var newvelocity Vec2 = GetDesiredVelocity()

var maxSteeringForce Vec2 = GetMaxSteeringForce()

var curmag float64 = curvelocity.Mag()
var diff float64 = newvelocity.Mag() - curmag

if math.Abs(diff) > maxSteeringForce {
    if diff > 0 {
        newvelocity = newvelocity.SetMag(curmag + maxSteeringForce)
    } else {
        newvelocity = newvelocity.SetMag(curmag - maxSteeringForce)
    }
}

The current implementation is incorrect as it does not take into account the changes of direction between the two velocities.

[xtuc]

Do we plan to support some kind of drag force? Even if null atm

Let's talk about that stuff tomorrow