-Clean up R_FakeFlat and update it to be in line with ZDoom 2.x

git-svn-id: http://odamex.net/svn/root/branches/rend32@3824 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Merge from trunk

git-svn-id: http://odamex.net/svn/root/branches/rend32@3826 b0bcef46-2027-0410-bd56-ab726e9a1d04