Reduced visual clutter on the zoomed-out world culture map.

On the world culture map, civs are now only treated as co-owners of
plots when they have >= 20% cultural ownership.

(Previously, having any culture whatsoever was enough, and so the map
was a complete mess.)

Thanks to Manujuanel for pointing out this problem.
Fixed a bug which messed up some of the calculations in AI_baseBonusVal

I was using the index, iJ, when I should have been using
'getPrereqOrBonuses(iJ)'.

Thanks to sjodster for spotting this mistake.
This is version 1.42b.

Updated the changelog.