Expose barycentric coord and triangle index

[ottah]

This change exposes the intersection's barycentric coord and the triangle index so they can be used to look up vertex attributes and texcoords.

[brookman]

Maybe I misunderstand something, but it seems that triangle_index contains only the first vertex index of the triangle. How can I get the other two vertex indices?

[aevyrie]

This should give you the triange index. In indexed geometry, you would look up the triangle with this index, and get the three vertices. In a non-indexed geometry the next three vertices should give you the triangle.