-Use ZDoom 1.23's sprite sorting comparison function, which will draw the taller sprite on top of the shorter sprite in when two sprites are at the same distance from the viewer. The previous comparison made such a case undefined, causing such sprites to flicker (See UDM3.WAD MAP31)

git-svn-id: http://odamex.net/svn/root/branches/rend32@3713 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix horizontal wall texture alignment.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3714 b0bcef46-2027-0410-bd56-ab726e9a1d04
-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