Add unit tests for rendering

[cochrane]

This part will take a bit longer, but should provide some clear advantages: One can verify that all shaders work, as well as posed models and special new features (like clothes #7 , texture packs #10 , new render groups #8 ). This should make it easier and quicker to iterate on graphic-related things.

It will be necessary to have some automatic way to generate new "expected" values, because if I do change things in an incompatible way, I don't want to have to generate all expected images again.

Not sure if it's possible and useful to simply compare screenshots; OpenGL doesn't really promise pixel-perfect accuracy.

[cochrane]

Implemented (without use of AppleScript). Comparing screenshots doesn't work, so every computer has to calculate its own set of "expected" images first and then keep them forever.