The face merging code in exact boolean made an assumption that
the tesselated original face was manifold except at the boundaries.
This should be true but sometimes (e.g., if the input faces have
self-intersection, as happens in the example), it is not.
This commit makes face merging tolerant of such a situation.
It might leave some stray edges from triangulation, but it should
only happen if the input is malformed.
Note: the input may be malformed if there were previous booleans
in the stack, since snapping the exact result to float coordinates
is not guaranteed to leave the mesh without defects.
This is the second try at this commit. The previous one had a typo
in it -- luckily, the tests caught the problem.
7c6d546f3a