private static Transform Convert(Matrix4x4 mat)
{
Vector3 T;
Matrix3x3 R;
Vector3 S;
mat.Decompress (out T, out R, out S);
var posA = new XnaVector2 (T.X, T.Y);
var rotA = new Mat22 (R[0], R[1], R[3], R[4]);
return new Transform (ref posA, ref rotA);
}