This changes how dirt blocks turn to dirt_with -grass, -dry_grass
or -snow.
Previously, dirt that was sunlit would turn to dirt_with_grass no
matter what, but this happened without any context, so you could
get green patches of dirt_with_grass in the middle of a savannah or
even desert.
Dirt no longer turns to covered dirt unless it's within 1 node from
another dirt_with_grass or dirt_with_dry_grass or dirt_with_snow.
This makes dirt_with_grass "growback" a lot slower, since it now only
happens on the edges, but it retains the context nicely now.
If there is any dirt with a grass or dry grass plant, or snow on top,
and enough light, we'll convert it sporadically to dirt_with_grass
or dirt_with_dry_grass or dirt_with_snow.
This allows us to plant grass of our choice in a large dirt patch,
or in a region where otherwise that type of grass is not present.
This used to be done by 2 abms, but I've combined them in to a single
ABM that is ordered to run with maximum efficiency, solving for the
most common outcome first before attempting more complex checks.
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