function normalize(out, a) {
  var x = a[0];
  var y = a[1];
  var z = a[2];
  var w = a[3];
  var len = x * x + y * y + z * z + w * w;
  if (len > 0) {
    len = 1 / Math.sqrt(len);
    out[0] = x * len;
    out[1] = y * len;
    out[2] = z * len;
    out[3] = w * len;
  }
  return out;
}
Normalize a vec4

@param {vec4} out the receiving vector
@param {vec4} a vector to normalize
@returns {vec4} out