fix csg build

[LinJiarui]

resolve the following issue:
when build 2 or more coplanar polygons, the original method maybe split all the polygons into the front node (or the back node), thus leads to stackoverflow.

[bhouston]

We are trying to make a centralized active repository here: https://github.com/csg-js/csg.js

[LinJiarui]

So, should I create a new pull request for the active repository?

[stefnotch]

This PR causes doesn't handle coplanar polygons correctly.
The following script shows this rather nicely:

var a = CSG.cube();
var b = CSG.cube({ center: [-0.25, 0, -0.25] });
a.setColor(1, 1, 0);
b.setColor(0, 0.5, 1);
return a.subtract(b);