resolveVelocity method
Implementation
@override
Offset resolveVelocity(int index, int total, Random rng) {
final a = angle + (rng.nextDouble() * 2 - 1) * spreadVariance;
final speed = minSpeed + rng.nextDouble() * (maxSpeed - minSpeed);
return Offset(cos(a) * speed, sin(a) * speed);
}