Add scene loader using "assimp" library

[snogglethorpe]

This library can load tons formats, and it's interface seems reasonable enough (generic scene descriptions are inevitably annoying, but this seems dealable).

It's huge though, so maybe add this as a dynamically loadable module via Lua.

One annoying thing ... it seems to use Blender definitions for materials, which are ... weird (e.g., Blender's Cook-Torrance model doesn't seem to actually resemble Cook-Torrance at all).