Fix T40405: Blender crashes on FBX export instantly.

Better fix than rBbef5cb3aa2e5a: consider edges between faces with opposed normals as sharp.

In fact, previous code was broken more deeply in this case (inconsistent normals across
a 'smooth fan') - some loop normals would even never be computed!

Fixing this is possible (even wrote it, actually), but this adds more complexity
to a piece of code that is already awfully complicated, *and* normals in that kind
of smooth fan do not make much sense anyway. So simpler and nicer results with
assuming sharp edges between such 'opposed' faces!

Note that there is some face (loop) ordering black magic at work here, added more comments
to try to explain how and why all this works.

As a bonus, we do not need to check for already computed loop normals anymore, since we
know each 'smooth fan' will be walked once, and only once.

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