Fixed segfault in R_FogFactor

An out of bounds array lookup was occurring in 64bit Debian, when on the "race" level of Jedi Academy, I believe that these changes to R_FogFactor fix this.

This was due to an issue with converting a float into an array index. I will not totally guarantee the correctness of this code, as I did not check all uses of this function, and the corresponding fog Table array.

I'd also like to add that you folks have done a great job patching up  this code-base. Being able to play Jedi Academy on linux is awesome, as this was one of my favourite games when I was younger.