Get *path* from igl::exact_geodesic

[fferri]

igl::exact_geodesic returns a scalar field. In order to get the actual shortest geodesic path, I have to combine the scalar field with info about mesh vertex/face connectivity, which I build manually from F (i.e. start from target, choose the connected vertex/face which has the lowest distance, and so on recursively).

It would be nice to have an example showing how to do that in a way that is idiomatic to libigl (or a igl::exact_geodesic overload which also returns the path).