Convert scenes to the new Lights format.

Namely, all light* properties of GlobalSettings are now in a
PointLight.

Add two new scenes, an open-space one (meshes.trinity) and a closed one
(boxed.trinity). The renderer defaults to the latter.

Also, fix the base property of a light to be called `color', instead of `col'.
Forgot to make the scene lighting changes to the HW9 scene set. Do it now.
Make RectLight vary its output accoring to cos(shadePoint, lightDir)

Since our RectLight was assumed to be shining in one hemisphere only (-y
canonical direction), there was a sharp boundary between lit and unlit
in boxed.trinity. It makes much more sense to have the light's output
dependant on the angle through which the rect light is "seen" from the
shade point: the more grazing angles we consider, the less the light.
Implement a cos(shadePointDir, lightDir) law. This makes the boundary smooth.