-Use R_ClipWallSegment from prboom-plus.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3715 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Don't attempt to round wallheights when projecting them onto the screen. This helps prevent seeing a thin band of ceiling texture at the bottom of closed doors.
-Update comments in r_segs.cpp and do some general code clean-up.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3716 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix a clipseg issue with IDL2013L.WAD MAP04 introduced in r3715.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3717 b0bcef46-2027-0410-bd56-ab726e9a1d04