function conjugate(out, a) {
  out[0] = -a[0];
  out[1] = -a[1];
  out[2] = -a[2];
  out[3] = a[3];
  return out;
}
Calculates the conjugate of a quat
If the quaternion is normalized, this function is faster than quat.inverse and produces the same result.

@param {quat} out the receiving quaternion
@param {quat} a quat to calculate conjugate of
@returns {quat} out