Replace vanilla sun and moon with more realistic one

[depressed-pho]

Which means we are probably going to stop using their textures. But for the moon it is very important to express its phase, which might or might not be possible.

[depressed-pho]

This turned out to be much harder than I expected for these reasons:

• The game apparently changes the alpha blending mode for the sun and the moon depending on the view angle, which causes a strange visual artifact that is hard to live with.
• No data about the moon phase is passed to the shader (as expected). The only data we can use is the texture coords, which I REALLY HATE to depend on.
• We can increase the brightness of the sky when we are looking at the sun, but we cannot change the brightness of the terrain, because shaders don't know where the sun is located at. Bright sky without bright terrain makes it look terribly bad.

[depressed-pho]

At the end of the day I decided to do it anyway. It's in Git master now. Not released yet though.