-Refactor common code to compare the floor and ceiling properties of two sectors int P_SectorFloorsMatch and P_SectorCeilingsMatch. Call those functions from R_StoreWallRange and R_EmptyLine.
-Make use of R_IsSkyFlat in R_StoreWallRange to properly handle more than one sky flat.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@4388 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix a crash when trying to calculate texture height for wall-tiers without textures.
-Simplify R_StoreWallRange a tad.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@4389 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix the drawing of masked-midtextures that overlap sprites on the screen.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@4390 b0bcef46-2027-0410-bd56-ab726e9a1d04