Add support for silhouette edges

[ianmackenzie]

Should be able to reuse much of the shadow edges logic to tag lines with their adjacent face normals, then only render silhouette edges

[ianmackenzie]

Similar to Mesh.shadow, might be able to have

Mesh.silhouette : Mesh coordinates a -> Silhouette coordinates

and then Scene3d.silhouette, Scene3d.withSilhouette etc.

[ianmackenzie]

Possibly useful: use trick described in http://www.terathon.com/gdc07_lengyel.pdf to adjust depth calculation, instead of using actual polygon offset