Take slopes into account in FOF wall cutoff in HWR_ProcessSeg

Fixes #192 (closed).

I also have a branch that puts the changes behind a cvar gr_foftest if you want to test and compare it live.

https://git.do.srb2.org/Hannu_Hanhi/SRB2/tree/ogl-fof-wall-slope-fix-test

Edit 10.1.2021:

I snuck in another fix in this branch, since it depends on the code in this merge request.

This new patch fixes fof side cutting when the floor or ceiling is sloped. The first screenshot in #190 (closed) shows this issue.

I haven't added this new fix to the comparison testing branch mentioned above. (ogl-fof-wall-slope-fix-test)