Ray casts which touch a TriMesh edge report intersection with a backface

This issue was originally reported on ncollide: https://github.com/dimforge/ncollide/issues/335

> Imagine Santa's sled knocking some snow off the top of a roof. It could be argued that no impact happened, or it could be argued that the impact happened with the edge. parry3d reports that Santa collided with the inside of the house.
>
> This is important for using ray casting to determine whether a point is inside a mesh. As per https://github.com/dimforge/ncollide/issues/317, I am casting a ray and checking whether it intersected with a backface. In the motivating example, a point nowhere near the mesh happens to cast a ray which skims a ridge in the mesh: due to this issue, a backface intersection is reported.

The issue is demonstrated here (on the `parry` branch): https://github.com/clbarnes/ncollide-issue-graze/tree/parry