implement auto-intersection and auto-union functions

[jimy-byerley]

Just like boolean operations between two meshes, these two would intersect faces and keep the most outer or inner possible envelope. But instead of working with a pair of input enveloppes, they would work with a unique non-envelope mesh and return an envelope for it

The main issue is how to compute efficiently the intersection of a ngon by several concurrent triangles

we may proceed this way:

• reuse the intersection searching and computing algorithms
• progressively compute edge intersections of all triangles by all others, always updating the triangles buffer and index (costly)
• reuse the envelope extraction by propagation, but starting from most inward or most outward triangle