Add Renderable instance for NurbsSolid
Make specifying camera position & field of view a little easier

There's still a lot of work to do.  This API is close to OpenGL, rather
than anything nice to work with.  But at least the features are exposed, now.
Merge branch 'threed' of github.com:bergey/diagrams-opengl into threed
Update to build against Diagrams HEAD

Mostly about use of Located,
and new Trail' / Loop / Line types