Fix slope inconsistency for 64bit

- Fixed: Slope logic would change between 32bit and 64bit due to float
math.
Fixed build on UNIX systems other than OS X
- Fixed: fixrtext isn't needed with Win64 builds.
Merge branch 'master' of https://github.com/rheit/zdoom into z_osx_clean
Merge branch 'z_osx_clean' of https://github.com/alexey-lysiuk/gzdoom into osx
- now that we don't have to care anymore about SVN-related merging problems, let's save the 3 global variables of the GL renderer unconditionally, even if the software renderer is running.