make decomposing leaves sometimes not create dirt.

add a 1/2 chance for decomposing leaves to disappear instead of becoming
dirt.

mods/bg_tree/decompose.lua

@@ -43,6 +43,10 @@ blockgame.register_increasing_abm({
 		return data.value >= 25
 	end,
 	action = function (pos, node, data)
-		minetest.set_node(pos, {name = "bg_terrain:dirt"})
+		if minetest.chance(2) then
+			minetest.set_node(pos, {name = "bg_terrain:dirt"})
+		else
+			minetest.remove_node(pos)
+		end
 	end,
 })