code: public Vector2f transformDirection(Vector2fc v, Vector2f dest) {
          dest.set(m00 * v.x() + m10 * v.y(),
                   m01 * v.x() + m11 * v.y());
          return dest;
      }
docs: Transform/multiply the given 2D-vector, as if it was a 3D-vector with z=0, by
  this matrix and store the result in <code>dest</code>.
  <p>
  The given 2D-vector is treated as a 3D-vector with its z-component being <code>0.0</code>, so it
  will represent a direction in 2D-space rather than a position. This method will therefore
  not take the translation part of the matrix into account.
  <p>
  In order to store the result in the same vector, use {@link #transformDirection(Vector2f)}.
  
  @see #transformDirection(Vector2f)
  
  @param v
  the vector to transform
  @param dest
  will hold the result
  @return dest