fbm2D

//! requires noise2D
fn fbm2D(p: vec2<f32>, octaves: i32) -> f32 {
  var value = 0.0;
  var amplitude = 0.5;
  var frequency = 1.0;
  for (var i = 0; i < octaves; i++) {
    value += amplitude * noise2D(p * frequency);
    amplitude *= 0.5;
    frequency *= 2.0;
  }
  return value;
}