title: Convert Eigen Affine3d to Affine2d
body: <p>I have an <a href="http://eigen.tuxfamily.org/dox/TutorialGeometry.html#TutorialGeoTransform" rel="nofollow">affine transform</a> in 3D and I wish to discard any z-axis information from. Is there a convenient way to convert from an <code>Affine3d</code> to and <code>Affine2d</code>?</p>