I must say I don't see the purpose of the new vsol_digital_curve_2d class.
Digital geometry already exists -- it was placed in a separate library
(vdgl) but that library conceptually belongs to vsol.
The main difference between the old vdgl_digital_curve and the new
vsol_digital_curve_2d is the fact that the new one uses refcounted
vsol_point_2d's as guide points instead of pairs of double.
Which makes it less useful for dense points (which is the main use of vdgl).
But with sparse guide points, I don't see a difference between the new
vsol_digital_curve_2d and the class vsol_polyline_2d.
Hence my question: do we need this new class? It is just causing
confusion with the existing digital geometry in vdgl (e.g., by its
replacement of cast_to_digital_curve()).