Make drop logic work better for non-8-step-plants. (#1544)

Using @paramat's formula.

mods/farming/api.lua

@@ -330,12 +330,16 @@ farming.register_plant = function(name, def)
 
 	-- Register growing steps
 	for i = 1, def.steps do
+		local base_rarity = 1
+		if def.steps ~= 1 then
+			base_rarity =  8 - (i - 1) * 7 / (def.steps - 1)
+		end
 		local drop = {
 			items = {
-				{items = {mname .. ":" .. pname}, rarity = 9 - i},
-				{items = {mname .. ":" .. pname}, rarity= 18 - i * 2},
-				{items = {mname .. ":seed_" .. pname}, rarity = 9 - i},
-				{items = {mname .. ":seed_" .. pname}, rarity = 18 - i * 2},
+				{items = {mname .. ":" .. pname}, rarity = base_rarity},
+				{items = {mname .. ":" .. pname}, rarity = base_rarity * 2},
+				{items = {mname .. ":seed_" .. pname}, rarity = base_rarity},
+				{items = {mname .. ":seed_" .. pname}, rarity = base_rarity * 2},
 			}
 		}
 		local nodegroups = {snappy = 3, flammable = 2, plant = 1, not_in_creative_inventory = 1, attached_node = 1}