Furnace: Fix being able to cook items without enough fuel

This was triggered when too much time had elapsed when timer was called.
Also, fix timer resolution giving free fuel time.

mods/default/furnace.lua

@@ -119,7 +119,7 @@ local function furnace_node_timer(pos, elapsed)
 	local fuel
 
 	local update = true
-	while update do
+	while elapsed > 0 and update do
 		update = false
 
 		srclist = inv:get_list("src")
@@ -134,13 +134,18 @@ local function furnace_node_timer(pos, elapsed)
 		cooked, aftercooked = minetest.get_craft_result({method = "cooking", width = 1, items = srclist})
 		cookable = cooked.time ~= 0
 
+		local el = math.min(elapsed, fuel_totaltime - fuel_time)
+		if cookable then -- fuel lasts long enough, adjust el to cooking duration
+			el = math.min(el, cooked.time - src_time)
+		end
+
 		-- Check if we have enough fuel to burn
 		if fuel_time < fuel_totaltime then
 			-- The furnace is currently active and has enough fuel
-			fuel_time = fuel_time + elapsed
+			fuel_time = fuel_time + el
 			-- If there is a cookable item then check if it is ready yet
 			if cookable then
-				src_time = src_time + elapsed
+				src_time = src_time + el
 				if src_time >= cooked.time then
 					-- Place result in dst list if possible
 					if inv:room_for_item("dst", cooked.item) then
@@ -149,6 +154,9 @@ local function furnace_node_timer(pos, elapsed)
 						src_time = src_time - cooked.time
 						update = true
 					end
+				else
+					-- Item could not be cooked: probably missing fuel
+					update = true
 				end
 			end
 		else
@@ -166,8 +174,7 @@ local function furnace_node_timer(pos, elapsed)
 					-- Take fuel from fuel list
 					inv:set_stack("fuel", 1, afterfuel.items[1])
 					update = true
-					fuel_totaltime = fuel.time + (fuel_time - fuel_totaltime)
+					fuel_totaltime = fuel.time + (fuel_totaltime - fuel_time)
-					src_time = src_time + elapsed
 				end
 			else
 				-- We don't need to get new fuel since there is no cookable item
@@ -177,7 +184,7 @@ local function furnace_node_timer(pos, elapsed)
 			fuel_time = 0
 		end
 
-		elapsed = 0
+		elapsed = elapsed - el
 	end
 
 	if fuel and fuel_totaltime > fuel.time then