tr_shader: linearize fog colors

[illwieckz]

Cherry-pick from #1730:

• WIP: tr_shader: linearize fog colors and fog image #1730

This one shouldn't be controversial.

One discussion possible is that we can do this conversion either in tr_shader.cpp, either in tr_bsp.cpp.

Fog shaders should be parsed when requested by the BSP code, there is no meaning outside of that, so the behavior should be the same.

[illwieckz]

After some thinking, I moved the conversion to tr_bsp. It's just a feeling and some presentiment, but I feel it in better way.

In fact, no other code in our code base converts colors at shader parsing, they all do it in code closer to their processing code than the shader code is, so I feel doing it that ways keeps the programming pattern more unified, and then it's easier to search for things by expecting where things can be in a more predictable way. That's the only reason why I went for preferring this.

[illwieckz]

This looks ready to me.

[slipher]

LGTM