Lights & Camera

This branch introduces Renderable instances for types added in https://github.com/diagrams/diagrams-lib/pull/114.  It should not be merged until that is.

The code below uses the FillColor attribute to set the diffuse lighting color.   Should we instead use a different attribute (SurfaceColor? DiffuseColor?), since the color in the rendered image is a combination of the surface color and the lighting?

I'm particularly interested in how much control people want over color / surface properties.  The currently-possible images have no highlights, and there is no ambient light, so shadows are too dark.  These should be easy to add later, but I'm not sure what properties are common, and which are idiosyncratic to POVRay.