-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
-Fix a ZDoom 1.22 off-by-one error in R_DrawColumnHorizP and reformat the function for readability.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3720 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Change uses of float to double in P_SetupSlope for consistency sake. Also do some minor code cleanup.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3723 b0bcef46-2027-0410-bd56-ab726e9a1d04