-Wrap the dc_XXXXX variables inside a drawcolumn_t struct and add a global instance 'dcol'.
-Wrap the ds_XXXXX variables inside a drawspan_t struct and add a global instance 'dspan'.
-Change the column/span shading functors to use drawcolumn_t or drawspan_t.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3987 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Remove the colormap parameter from the shading functors' constructors since the colormap can be obtained from the drawcolumn_t/drawspan_t parameter.


git-svn-id: http://odamex.net/svn/root/branches/rend32@3988 b0bcef46-2027-0410-bd56-ab726e9a1d04
-The slope span colormap pointer was not being properly incremented as the slope span was rasterized in r3988. Fixed.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3989 b0bcef46-2027-0410-bd56-ab726e9a1d04