SimplexNoise constructor

SimplexNoise([Random r ])

Implementation 

SimplexNoise([math.Random r]) {
  r ??= new math.Random();
  final List<int> p =
      new List<int>.generate(256, (_) => r.nextInt(256), growable: false);
  _perm = new List<int>.generate(p.length * 2, (int i) => p[i % p.length],
      growable: false);
  _permMod12 = new List<int>.generate(_perm.length, (int i) => _perm[i] % 12,
      growable: false);
}