Tests: Add tests to validate triangle splitting

[gkjohnson]

Requirements

• Move triangle splitting function from three-mesh-bvh to this repo for ease of updating (Bring the triangle clipping / intersection functions into this repo #70)
• Add series of jest tests for validating found edges and correct results
• Add series of jest tests for validating correct splitting of triangles (TriangleSplitter)
• See how viable it is to procedurally

Notes

• Ensure parallel, coplanar triangles are tested
• Ensure identical triangles are tested
• Ensure vertex-intersecting triangles are tested
• Ensure corner cases like overlapping vertices, edges are tested
• Ensure identical triangles are tested

[agviegas]

Hey, we are offering a bounty of $250 for this issue. Whoever solves it, will get that money. 😎

[HugoGresse]

Currently having a triangle issue when exporting as STL for 3D printing, do you think it's related?

Is there some workaround with the lib or are you aware of another great library for Three.js CSG?

[gkjohnson]

It's known that there are cases where the CSG operations fail to produce a solid mesh. At some point I want to change the approach used in the triangle clipping algorithm to something that should be a bit more robust and faster but there are other priorities at the moment. Even so, though, I don't believe there's a perfect solution when relying on floating point math so there will still be cases where this fails.

If this is for professional work and you'd like to fund and prioritize these kinds of improvements I'm happy to talk about that.