Use double precision in R_StoreWallRange sloped seg culling calculations (Fixes #434)

Fixes culling issues in CEZ2 skybox. See discussion in #434 (closed) for more information.

I think the map itself is already getting a separate fix for it but this MR should fix the underlying issue in the renderer, if it needs to be fixed.

I also changed the "TODO: convert to FPU" comments to say "fixed point" instead of "FPU". Apparently that was the intention of the comments, so I changed it in order to avoid readers being confused. https://en.wikipedia.org/wiki/Floating-point_unit

srb2win_sw-culling-inaccuracy.exe

Edit: Added build of the fix.