[Mapping] SkinningMapping: compute weights from position

[EulalieCoevoet]

By default, the weights of the SkinningMapping are computed w.r.t the rest position of the input model.
This PR adds an option to compute them from the position instead.

For exemple, I'm using this with pre-bent concentric tubes: the rest position is curved, while the visual model and position at start are straight.

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