The code added to postprocess the manifold result to remove
extra vertices introduced on edges had a logic bug in it.
It would sometimes dissolve a vertex from a triangle.
I thought that would be very rare and just repeated a vertex to
fill out the triangle, but that leads to a mesh that doesn't validate.
Anyway, my logic to prevent a single vertex from being dissolved from
a triangle was wrong (if the triangle was seen second while looking).
I fixed that, and then the even rarer case of two vertices being
dissolved from a quad showed up. I have prevented all such cases
by a loop that disables all vertex dissolving in faces where it
would leave less than three vertices.
Also added an assert (so, debug-mode only) that the mesh produced
by manifold boolean is valid.
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