this type is needed for edges that are part of the input constrained
edges property map, and are incident to a face to be remeshed only by a
vertex.
These edges contribute to creating corner vertices (incident to >= 3
constrained edges), but should still not be considered as patch borders
without dealing with these "isolated constraints", the corner vertices that
are incident to 2 patch borders and 1 of these edges was not considered
as a corner but as a simple patch border vertex, hence it could be
moved/deleted by a collapse or relaxation steps
Jane Tournois
d61cf58f2f