Remove dependency on trimesh

[clbarnes]

Trimesh is currently used for optional mesh validation/ fixing. Ideally would eventually show up in ncollide dimforge/ncollide#318

Currently handled by trimesh:

• Winding
• Holes

Other possible validations:

• Self-intersections
• Zero-area triangles
• Degenerate or unused vertices, triangles

[clbarnes]

1. Degenerate vertices (duplicates in vertices)
2. Unused vertices (any vertices not referred to in triangles)
3. Inconsistent winding (parallel edges); could also be edge-to-edge self-intersection
4. Holes (edge with 0 antiparallel partners)
5. Zero-area triangles (given no degenerate vertices, triangle has >=2 vertices at the same index)
6. Degenerate triangles (given consistent winding, two triangles have the same vertices)
7. Incorrect winding (given consistent winding, ray cast from outside the AABB at an arbitrary triangle hits a backface)

Self-intersections exist when an edge intersects with a triangle other than those it ought to?