Make the Rect light's output dependant on the lamp size.

This, making the lamp larger, without changing its color, would now make
the scene brighter. This formulation is now consistent with the path tracing algorithm.
Two scenes, to be used in demonstrations of the path tracing GI algorithm.

smallpt.trinity:     recreation of Kevin Beacon's 'smallpt' path-tracer example.
cornell_box.trinity: a Cornell Box scene, converted in our format by
                     Ivaylo Dankolov of the RTBG2011-2012 course.