refactor sapling support checking.

saplings now only check for groups, not specific nodes, when determining
whether they are supported. the previously checked nodes are now in the
new group that's being checked.

mods/bg_tree/grow_sapling.lua

@@ -9,8 +9,10 @@ local leaves_growing = modname .. ":leaves_growing"
 
 
 local function supports_sapling (name)
-	if name == log_alive or name == root_alive then return true end
-	return minetest.get_item_group(name, "dirty") > 0
+	return blockgame.item_matches(name, {
+		"group:supports_sapling",
+		"group:dirty",
+	})
 end
 
 -- this commented-out function will be used when roots start affecting tree growth.
@@ -39,7 +41,7 @@ blockgame.register_increasing_abm({
 	id = modname .. ":grow_sapling",
 	label = "grow sapling",
 	nodenames = {sapling},
-	neighbors = {"group:dirty", log_alive},
+	neighbors = {"group:supports_sapling", "group:dirty"},
 	interval = 15,
 	chance = 4,
 	rate = function (pos, node, data)

mods/bg_tree/node.lua

@@ -19,6 +19,7 @@ blockgame.register_node("root_alive", {
 	description = "Growing Root",
 	groups = {
 		woody = 1,
+		supports_sapling = 1,
 	},
 	drop = modname .. ":root",
 })

mods/bg_tree/node_log.lua

@@ -28,6 +28,7 @@ reg_log("log", {
 reg_log("log_alive", {
 	groups = {
 		planty = 1,
+		supports_sapling = 1,
 	},
 	texture = "log",
 	drop = modname .. ":log",