-Fix a crash in rt_copycols when r_drawflat is enabled.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3718 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Add drawing function pointers R_FillColumn and R_FillSpan and implement R_FillColumnD and R_FillSpanD for 32bpp mode. This should hopefully restore r_drawflat functionality for 32bpp.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3719 b0bcef46-2027-0410-bd56-ab726e9a1d04