#82 Inset: prevent inverse/negative inset

[Dominique B.]

If you inset to the extreme, faces go to the other side
- flips somehow. I don't think that this is something a
modeller ever wants. So might be a good addition to
block the inset-operation at the minum scale - before
the verts go over the other side of the mesh (over the
middle or so).

If serveral faces are involved in the inset operation,
the operation should not halt for every face when one
of them reaches the minimum - it should only stop the
inset operation for the faces that reached the minimum
(i.e. verts touching the middle of the face or the
middle of all moving verts is within a treshhold range
of all concerned verts).

Hope I am expressing myself not tooooo unclear ;)