Apply rotation about the Z axis to align the local <code>+X</code> towards <code>(dirX, dirY)</code>.
<p>
If <code>M</code> is <code>this</code> matrix and <code>R</code> the rotation matrix,
then the new matrix will be <code>M * R</code>. So when transforming a
vector <code>v</code> with the new matrix by using <code>M * R * v</code>, the
rotation will be applied first!
<p>
The vector <code>(dirX, dirY)</code> must be a unit vector.
@param dirX
the x component of the normalized direction
@param dirY
the y component of the normalized direction
@return this
Apply rotation about the Z axis to align the local <code>+X</code> towards <code>(dirX, dirY)</code>. <p> If <code>M</code> is <code>this</code> matrix and <code>R</code> the rotation matrix, then the new matrix will be <code>M * R</code>. So when transforming a vector <code>v</code> with the new matrix by using <code>M * R * v</code>, the rotation will be applied first! <p> The vector <code>(dirX, dirY)</code> must be a unit vector.
@param dirX the x component of the normalized direction @param dirY the y component of the normalized direction @return this