make unsupported leaves die & fall.

leaves that aren't in contact with living logs, or are not in contact
with any supported leaves, or are too far away from living logs, will
now die and fall down.

mods/bg_tree/node_leaves.lua

@@ -1,9 +1,26 @@
 local modname = minetest.get_current_modname()
 
 local function leaves_fall_check (pos, node)
-	return blockgame.every(blockgame.vector.get_neighbors(pos), function (pos)
-		return minetest.get_item_group(minetest.get_node(pos).name, "supports_leaves") == 0
-	end)
+	local is_supported = false
+
+	blockgame.flood_fill(pos, function (pos, distance)
+		local nodename = minetest.get_node(pos).name
+
+		if blockgame.item_matches(nodename, {"group:supports_leaves"}) then
+			is_supported = true
+			return false
+		end
+
+		if blockgame.item_matches(nodename, {"group:extends_leaves_support"}) then
+			return true
+		end
+	end, 3)
+
+	if not is_supported then
+		minetest.set_node(pos, {name = modname .. ":leaves"})
+	end
+
+	return not is_supported
 end
 
 local function reg_leaves (name, def)
@@ -17,7 +34,7 @@ local function reg_leaves (name, def)
 
 	def.groups = blockgame.underride(def.groups, {
 		planty = 1,
-		supports_leaves = 1,
+		extends_leaves_support = 1,
 		air_flowable = 1,
 		can_fall = 1,
 	})

mods/bg_tree/node_log.lua

@@ -20,7 +20,11 @@ local function reg_log (name, def)
 	blockgame.register_node(modname .. ":" .. name, def)
 end
 
-reg_log("log")
+reg_log("log", {
+	groups = {
+		supports_leaves = 0,
+	},
+})
 reg_log("log_alive", {
 	groups = {
 		planty = 1,