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 {ReadonlyQuat} a quat to calculate W component of
@returns {quat} out