improve `flood_fill` api function.

make `flood_fill` able to be interrupted if its callback returns false;
the meaning that was previously indicated by this return value is now
indicated by nil instead.

update code that uses this function to account for this change.

mods/bg_api/util_node.lua

@@ -58,7 +58,9 @@ function blockgame.flood_fill (start, step, max_range)
 	local matches = {}
 	local rejected = {}
 
-	while #queue > 0 do
+	local cancel = false
+
+	while #queue > 0 and not cancel do
 		local current = table.remove(queue, 1)
 		local pos = current.pos
 		local range = current.range
@@ -66,14 +68,18 @@ function blockgame.flood_fill (start, step, max_range)
 		local hash = minetest.hash_node_position(pos)
 
 		if not visited[hash] and range <= max_range and step(pos, range) then
-			table.insert(matches, pos)
+			local step_result = step(pos, range)
 
-			local neighbors = blockgame.vector.get_neighbors(pos)
-			for _, p in pairs(neighbors) do
-				table.insert(queue, { pos = p, range = range + 1 })
-			end
-		else
-			table.insert(rejected, pos)
+			if step_result then
+				table.insert(matches, pos)
+
+				local neighbors = blockgame.vector.get_neighbors(pos)
+				for _, p in pairs(neighbors) do
+					table.insert(queue, { pos = p, range = range + 1 })
+				end
+			elseif step_result == nil then
+				table.insert(rejected, pos)
+			else cancel = true end
 		end
 
 		visited[hash] = true

mods/bg_tree/decompose.lua

@@ -70,8 +70,6 @@ blockgame.register_increasing_abm({
 				score = score + math.floor(decompose_node_scores[name] / distance)
 				return true
 			end
-
-			return false
 		end, 4)
 
 		return data.value + score