Brighter sun faces

Intended as a suggestion, please reject if the idea is bad.

I noticed in MineCraft that the environment looks a lot more beautiful and real when there's a brightness difference between block faces. Minetest already has brighter top surfaces, so I also made sides facing the sun / moon axis a bit brighter. I think it gives a lot more feel and realism until we'll have hardware lighting at last.

Note that I only tested the shader version, since for some reason the C++ / non-shader topside brightening doesn't work.