Add surfaceFogVars option for shaders

[zturtleman]

trafficstars

skyFogVars allow having exponential fog on the sky. (Last parsed skyFogVars effects all sky shaders.)

surfaceFogVars would all setting per-shader fog vars (exp and linear). Useful for having multiple fogged skies and having fog effect on random shaders for whatever reason. It would over-ride global/sky fog.