[art] [maps] Replace large grass fields on Savanna & Nubia biomes
They make problems on several maps where they either float (e.g. harbor.js) as they are placed to close to a cliff/ on a hill or they cover way too much (e.g. D4699 / wild_lake.js).
Related: https://code.wildfiregames.com/D4699#199688
one could adjust all maps, but it is so much easier to just change the actors here.
accepted by: @wowgetoffyourcellphone
Differential revision: https://code.wildfiregames.com/D4701