-Add a function R_GenerateVisSprite to reduce common sprite & particle projection code.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3830 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Clip sprites and particles closer than 1 map unit to prevent integer overflows when calculating the colormap to use.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3831 b0bcef46-2027-0410-bd56-ab726e9a1d04