Add Exponential Shadow Maps. Add Cascade Shadow Maps

[akira-cn]

Might be we should make shadow a bit more smooth.

[gordonnl]

Yes! I totally agree. There are quite a few different approaches, each with different pit-falls, and from what I understand they're all quite expensive compared to the current, basic implementation.

From the quick research I've done earlier, I'd like to see an exponential shadow map implementation - it seems to have nice results for relatively few side-effects.

And on the same topic, I'd also love to see an implementation of cascade shadow maps (for larger scenes)!