-Change the masked midtexture seg renderer to use the TextureManager for its source. Now flats can be used for masked midtexures.
-Masking still needs to be addressed with additional column drawing functions.
-The sky rendering also uses TextureManager.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@3983 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Use TextureManager for the source for sprite rendering as well.
-Finally the ZDoom 4 column rendering scheme can be removed.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@3984 b0bcef46-2027-0410-bd56-ab726e9a1d04