function multiply(out, a, b) {
  var ax = a[0],
      ay = a[1],
      az = a[2],
      aw = a[3];
  var bx = b[0],
      by = b[1],
      bz = b[2],
      bw = b[3];

  out[0] = ax * bw + aw * bx + ay * bz - az * by;
  out[1] = ay * bw + aw * by + az * bx - ax * bz;
  out[2] = az * bw + aw * bz + ax * by - ay * bx;
  out[3] = aw * bw - ax * bx - ay * by - az * bz;
  return out;
}
Multiplies two quat's

@param {quat} out the receiving quaternion
@param {quat} a the first operand
@param {quat} b the second operand
@returns {quat} out