gradCoord2D function
Implementation
@pragma('vm:prefer-inline')
double gradCoord2D(int seed, int x, int y, double xd, double yd) {
IntX hash = Int32(seed);
hash ^= _xPrime * x;
hash ^= _yPrime * y;
hash = hash * hash * hash * 60493;
hash = (hash >> 13) ^ hash;
final g = gradient2d[hash.toInt() & 7];
return xd * g.x + yd * g.y;
}