TriangleSplitter: Enable symmetrical clipping along connected edges

[gkjohnson]

Related to #49

Requirements

• Add a variation of TriangleSplitter that will clip triangles such that edges are symmetrically split so the edges line up and can be connected in the HalfEdgeMap.

Approach

• When clipping triangles construct polygons from connected loops of edges from the edges derived from triangle intersections.
• Once all polygons have been found from the split determine whether they're inside or outside the model using the existing determination method (raycasting).
• For the polygons to keep run the three.js built-in Earcut method to triangulate the polygon to add to the final geometry.

Notes

• Alternatively the half edge structure could be redesigned to not require a 1:1 relationship on edges. Instead if triangles connect at T-junctions the relationships can be 1 to many, though this would be significantly more complicated and require ensuring the full length of the edge is accounted for.
• Polygon triangulation:
  • https://algo.kaust.edu.sa/Documents/cs372l05.pdf
  • https://en.wikipedia.org/wiki/Polygon_triangulation
  • https://github.com/mrdoob/three.js/blob/master/src/extras/Earcut.js

[agviegas]

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