- Add new function for the new slope calculations.

Fixed also two MSVC warnings.
- Oops comment typo.
- Added flags for A_Fade functions:
- FTF_REMOVE: Removes the actor when the alpha hits a certain level.
- - A_FadeIn - 1.0
- - A_FadeOut - 0.0
- - A_FadeTo - Alpha target level reached
- FTF_CLAMP: Automatically fixes the alpha so it won't leave the range [0.0, 1.0].
Merge branch 'fade' of https://github.com/MajorCooke/zdoom