umask, vmask, ushift, and vshift don't need to be calculated for each span of a plane. Calculate them once and store them in drawspan_t.
-Remove texturewidthbits and textureheightbits from drawspan_t since they were only used to calculate umask and vmask.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@4382 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Move the tests for solid line segs and empty line segs only used for triggers into their own functions, called from R_AddLine.

git-svn-id: http://odamex.net/svn/root/branches/texmanager@4383 b0bcef46-2027-0410-bd56-ab726e9a1d04