#89 Test Suite

It would be very useful to have an automated test suite for running
regression tests, to generate a set of current images and compare
them to set of reference images to identify problems (e.g., a
change to the renderer which causes an unexpected side effect).

This could also be turned into a performance testing tool, as a test
harness can easily execute performance tests along with
regression tests.

We have the beginnings of input data for this in SDK/ModelsTest,
but really need a cross-platform app that can construct a scene,
capture an image, and compare that image to a reference copy.