More powerful shape primitives

[unixpickle]

There are a bunch of primitive shapes (spheres, cylinders, etc.) implemented as Solids. These primitives could be made to implement other interfaces, like Collider and SDF as well. They should then be moved from solid.go to primitives.go. Along with this change, all of the Collider methods on Triangle should be moved to the rest of the Triangle methods in primitives.go.

This will remove a lot of unnecessary Object implementations in render3d, which basically implements a Collider for spheres, cylinders, and planes.