function calculateW(out, a) {
  var x = a[0],
      y = a[1],
      z = a[2];

  out[0] = x;
  out[1] = y;
  out[2] = z;
  out[3] = Math.sqrt(Math.abs(1.0 - x * x - y * y - z * z));
  return out;
}
Calculates the W component of a quat from the X, Y, and Z components.
Assumes that quaternion is 1 unit in length.
Any existing W component will be ignored.

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