use mapbox/earcut library for efficient earclipping

[andreasplesch]

This PR replaces the earclipping algorithm with a more efficient earclipping library available here:
https://github.com/mapbox/earcut
The main advantage is handling of corner cases and large polygons.
The dedicated library is compact and included directly in EarClipping.js but it could be moved to a separate file.
There is some required remapping of input and output to and from the library which could be optimized by directly using the provided x3dom linkedlist.
The library also deals with holes in polygons if needed elsewhere (fonts?) at some point.
This user script allows for testing:
https://gist.githubusercontent.com/andreasplesch/42d6405d0de8cc2472b8/raw/x3domEarcut.user.js

[ajcyeh]

Have included this script for testing from https://vrmath2.net/VRM2/?cm=old
Both extrusions below are fast and accurate.
https://vrmath2.net/sites/default/files/user/u3/logo/peano_extrusion.logo
https://vrmath2.net/sites/default/files/user/u3/logo/Koch_snowflake_extrusion.logo

[andreasplesch]

Thanks again Andy for testing.

All these tests use the new PR:

#379, http://andreasplesch.github.io/x3dom/earclipping/SriCampus012.xhtml
#341 , http://andreasplesch.github.io/x3dom/earclipping/x3dom_extrusion.html

http://andreasplesch.github.io/x3dom/earclipping/arch.xhtml

The concave regression test
http://andreasplesch.github.io/x3dom/earclipping/concave.html

Complicated extrusions
http://andreasplesch.github.io/x3dom/earclipping/extrusion.html

[mlimper]

Thanks!

[andreasplesch]

Ok. I will let Vladimir, the author of earcut, know as well.

mapbox/earcut#44