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.

I only tested the shader version since for some reason the C++ / non-shader topside brightening won't work.

http://i42.tinypic.com/9an53q.png

http://i39.tinypic.com/2mgns3l.png