Patch: Add per-triangle attribute

[JensRestemeier]

I needed to re-map triangle/vertex attributes from the source meshes to the result mesh, and the easiest way seemed to be a per-triangle integer. In the easiest case this can be a material index, or you can store a source triangle id to look up the original triangle/vertices after the operation to interpolate vertex attributes.

0001-Added-per-triangle-attribute-to-allow-re-mapping-of-.patch.txt