Unnatural convolution

By using a simple convolution filter, with a small 5x5 kernel running on a fragment shader, one can obtain a nicer and softer shadows.

This trick was invented by Anirudh Shastry; it's not physically correct, it's prone to artifacts (and that's why I'm gonna use stencils) but it's good to see and pretty fast. Moreover, I didn't have to move the whole rendering pipeline in one shader, which is good.