Working towards instancing and transforms.

This compiles, but currently segfaults when trying to render a scene.
The changes made so far are:
- Lights are now considered objects, just like anything else in the scene.
- An Assembly class has been defined, which is a self-contained set
  of objects and shaders.  Assemblies will be the building block of
  instance- and transform-hierarchies.  In this code they are currently
  being intentionally mis-used, just to get things running.
Now it doesn't crash when rendering. Yay!

There are some weird rendering artifacts, though.  Need to track those down.