Make fog consistent between shaders

[Alayan-stk-2]

We have three different shaders that all seem to be using different formulas and obeying different parameters for fog: pointlightscatter.frag, sp_transparent.frag, combine_diffuse_color.frag

Some fog parameters, like the max fog, are ignored by some of those shaders.

I haven't investigated too deeply, but it appears that they were developed independently without properly seeking consistency, and this can lead to some strange visual effects (for example, strong haze from point lights even if the track is otherwise not too foggy).