mmmm... I think this is used by `composition` not for `d_dp`.

The thing is that the composition code is shared by `GlobalMD` and `OrthoMD` and the jacobian of its respective global transforms comes from 'Similarity' which needs `points` to be computed and from `Similarity2dInstanceModel` for which `d_dp` is just a property expecting no `points`. We need a shared call for this `d_dp` which always expects `points` as an argument  and  in `GlobalMD` passes them to the `Similarity.d_dp()` method and in in `OrthoMD` ditches them and simply asks for the `d_dp` property of `Similarity2dInstanceModel`.