GC reports nonmanifold mesh when mesh is just not oriented consistently

Start by locating the GC validation that reports a mesh as nonmanifold and reproduce the case where the mesh is manifold but faces are inconsistently oriented. Decide whether the intended scope is revised error text, explicit orientation detection, or an orientFaces() method; done means the selected behavior distinguishes or resolves this case without misreporting it.

GC will report that a mesh is nonmanifold even in cases where it is manifold, but the faces are not consistently aligned.

Level 1 fix

A quick improvement would just be to change error reporting to something like:

Mesh is either nonmanifold, or faces are not oriented consistently.

Level 2 fix

A bigger fix would be to detect that faces are inconsistently oriented, and report this explicitly:

Mesh is manifold, but faces are not consistently oriented.

Level 3 fix

The ultimate fix would be to actually fix the normal orientation, if the mesh is manifold and orientable. There are some design decisions here (e.g., which of the two orientations should we pick?) but generally this should be pretty straightforward.

We could also implement this as a general-purpose method like orientFaces(), which is probably useful in its own right. Then the user can be asked to call this method explicitly:

Mesh is manifold, but faces are not consistently oriented.  Please call orientFaces() before trying to build a ManifoldSurfaceMesh.

This may be better than automatically orienting someone's faces, which might violate the principle of least surprise.