Useage : "...[.... csg*]" equals "current mesh -CSG- the mesh in the braces".
Keywords :  CsgUnion<csgu>, CsgSubstract<csgs>, CsgAnd<csga>, CsgXor<csgx>
TODO : cleanup useless code.
TODO : Some bugs are still present, some face that disappear should not.
TODO : Correct epsilon useage (see geometry files).
TODO : Coplanar face are not handled -at all-.
TODO : Vertex count goes through the roof in case of a Xor.