[Mesh] Avoid false negatives during self-intersection checks

[AjinkyaDahale]

Fixes issue #4732.

Within the code, a different algorithm is used for intersection checks when
faces are found to be coplanar. That algorithm does not return the "intersection
line", which is used to remove false-positives. This however results in
false-negatives.

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[AjinkyaDahale]

Possibly not complete since performing the check sometimes makes the mesh disappear temporarily until "repaired". Please test with the STL file provided in forum.

[chennes]

I can confirm that this compiles and runs, and both fixes the self-intersection problem, and introduces the "hidden mesh" problem.

[AjinkyaDahale]

Marking as ready for review since @yorikvanhavre mentioned mentioned this somewhere (though I don't recall where)

[yorikvanhavre]

Okay let's merge then. The hidden mesh bug doesn't seem critcal. (Indeed I have a déja-vu impression of having written that already somewhere, but can't find where 😆 )

[luzpaz]

Please add to the 0.20 Release Page

[AjinkyaDahale]

Okay let's merge then. The hidden mesh bug doesn't seem critcal. (Indeed I have a déja-vu impression of having written that already somewhere, but can't find where laughing )

@yorikvanhavre: confirmed that I have an email where this was a reply. I guess you deleted the comment sometime.

[AjinkyaDahale]

Please add to the 0.20 Release Page

Added to thread because I don't have edit access.

[AjinkyaDahale]

Guess I did have that edit access after all. Made the changes.