MeshCutter: Add inner faces.

[gonnavis]

Fix: webaverse/app#2618
Demo: https://raw.githack.com/gonnavis/metaverse-utils/898006d9b6555ded898b0022e0022d4531a31e68/mesh-cut/test.html

Trying to add inner faces at cut plane:

Tried to think of solutions myself, such as line sweep points, but felt that there were various problems that could not be solved.
So searched and found the Delaunay triangulation algorithm, and found a repo https://github.com/mapbox/delaunator .

[gonnavis]

Successfully using delaunator to generate inner faces base on cut-edge points ( pointsInner ).

[gonnavis]

Added uv to inner faces.
Though the inner faces' uvs should be logically correct, but the result is not what wanted.
Such as these four points logically really all has uv (0.5, 0.5), but resulting the center area all sampled the white line pixel in the center of the image.
Need to try uv remapping or triplanar texture.

[gonnavis]

Though delaunator works very well on generate faces from points, but it can't know which triangles are outer of the original model alone, i.e., can only output Convex result.

So I tried to store linesInner and compare to generated lines to filter out outer triangles.
Done the first outer layer, need recur to filter out all.

---EDIT---
May use some existing Constrained Delaunay Triangulation solution:
https://www.mathworks.com/help/matlab/math/creating-and-editing-delaunay-triangulations.html
mapbox/delaunator#9
https://github.com/mapbox/earcut

[gonnavis]

Successfully filtered out outer triangles:

Currently, still using the comparing-lines filter out method I came up with, works fine for non-cross geometries.
Todo:

• ✔️Support arbitrary cutting plane ( currently only supporting generate inner faces of Z-axis cutting plane ) .
• Performance: Prevent redundant codes.

May try existing Constrained Delaunay Triangulation solutions afterwards:
https://www.mathworks.com/help/matlab/math/creating-and-editing-delaunay-triangulations.html
mapbox/delaunator#9
https://github.com/mapbox/earcut

[gonnavis]

OK! Generate inner faces.
Demo: https://raw.githack.com/gonnavis/metaverse-utils/b01fa5ab6a3098c88b21dfa19b156ad2c8519c14/mesh-cut/test.html

Todo:

• UV remapping.
• Performance: Prevent redundant codes.

[gonnavis]

This PR was originally written for the needs of Aray's OT project, but in the end Aray achieved the effect with 3D Voronoi, without using MeshCutter.

The functionality of this PR has been implemented so far, but many optimizations and tests are still needed. It can be reopened if necessary.