Change how dirt turns to dirt_with_(something)

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.

mods/default/functions.lua

@@ -353,32 +353,69 @@ minetest.register_abm({
 
 
 --
--- Grass growing on well-lit dirt
+-- Convert dirt to something that fits the environment
 --
 
 minetest.register_abm({
 	nodenames = {"default:dirt"},
-	neighbors = {"air"},
+	neighbors = {
+		"default:dirt_with_grass",
+		"default:dirt_with_dry_grass",
+		"default:dirt_with_snow",
+		"default:grass_1",
+		"default:grass_2",
+		"default:grass_3",
+		"default:grass_4",
+		"default:grass_5",
+		"default:dry_grass_1",
+		"default:dry_grass_2",
+		"default:dry_grass_3",
+		"default:dry_grass_4",
+		"default:dry_grass_5",
+		"default:snow",
+	},
 	interval = 6,
 	chance = 67,
 	catch_up = false,
 	action = function(pos, node)
+		-- Most likely case, half the time it's too dark for this.
 		local above = {x = pos.x, y = pos.y + 1, z = pos.z}
-		local name = minetest.get_node(above).name
-		local nodedef = minetest.registered_nodes[name]
-		if nodedef and (nodedef.sunlight_propagates or nodedef.paramtype == "light") and
-				nodedef.liquidtype == "none" and
-				(minetest.get_node_light(above) or 0) >= 13 then
-			if name == "default:snow" or name == "default:snowblock" then
-				minetest.set_node(pos, {name = "default:dirt_with_snow"})
-			else
-				minetest.set_node(pos, {name = "default:dirt_with_grass"})
+		if (minetest.get_node_light(above) or 0) < 13 then
+			return
+		end
+
+		-- Look for likely neighbors.
+		local p2 = minetest.find_node_near(pos, 1, {"default:dirt_with_grass",
+				"default:dirt_with_dry_grass", "default:dirt_with_snow"})
+		if p2 then
+			-- But the node needs to be under air in this case.
+			local n2 = minetest.get_node(above)
+			if n2 and n2.name == "air" then
+				local n3 = minetest.get_node(p2)
+				minetest.set_node(pos, {name = n3.name})
+				return
 			end
 		end
+
+		-- Anything on top?
+		local n2 = minetest.get_node(above)
+		if not n2 then
+			return
+		end
+
+		local name = n2.name
+		-- Snow check is cheapest, so comes first.
+		if name == "default:snow" then
+			minetest.set_node(pos, {name = "default:dirt_with_snow"})
+		-- Most likely case first.
+		elseif name:sub(1, 13) == "default:grass" then
+			minetest.set_node(pos, {name = "default:dirt_with_grass"})
+		elseif name:sub(1, 17) == "default:dry_grass" then
+			minetest.set_node(pos, {name = "default:dirt_with_dry_grass"})
+		end
 	end
 })
 
-
 --
 -- Grass and dry grass removed in darkness
 --