When n-gons share vertices, their triangulation can be non-manifold,
even if the original mesh is manifold.
The UV Unwrapper does not currently work with non-manifold meshes.
This workaround attempts to modify the triangulation of n-gons in
the UV unwrapper to preserve the manifold property.
This change replaces the previous fix for quads, and extends it
to all n-gons.
See T84078 as motivation for this change.
Differential Revision: https://developer.blender.org/D16521
644afda7eb