add retessellation to stl_boolean

[jallwine]

The meshes that are output from stl_boolean are generally non-manifold. Finish porting the retessellation parts of CSG.js to fix this (I think).

[jallwine]

d854d84 ensures stl_boolean outputs manifold meshes. Problems occur with highly detailed meshes. Perhaps retessellation could help in some of those cases? I think it would at least lower number of triangles in the final output.