Do not set fire to objects that are already burning.

Changes to mcl_burning.set_on_fire():
* Add logic that only updates the burn time, but skips adding fire
  entities to objects that are already burning.
* Condense code a little, remove single use variables.
* Add a comment to a questionable piece of code.
* Add comments to the function.

mods/ENTITIES/mcl_burning/api.lua

@@ -58,13 +58,24 @@ function mcl_burning.update_hud(player)
 	end
 end
 
+-- Sets and object state as burning and adds a fire animation to the object.
+--
+-- Parameters:
+--   obj - may be a player or a lua_entity;
+--   burn_time - sets the object's burn duration;
+--
+-- If obj is a player, adds a fire animation to the HUD, if obj is a
+-- lua_entity, adds an animated fire entity to obj.
+-- The effective burn duration is modified by obj's armor protection.
+-- If obj was already burning, its burn duration is updated if the current
+-- duration is less than burn_time.
+-- If obj is dead, fireproof or a creative player, this function does nothing.
+--
 function mcl_burning.set_on_fire(obj, burn_time)
 	if obj:get_hp() < 0 then
 		return
 	end
 
-	local storage = mcl_burning.get_storage(obj)
-
 	local luaentity = obj:get_luaentity()
 	if luaentity and luaentity.fire_resistant then
 		return
@@ -85,36 +96,37 @@ function mcl_burning.set_on_fire(obj, burn_time)
 				end
 			end
 		end
-
 		if max_fire_prot_lvl > 0 then
 			burn_time = burn_time - math.floor(burn_time * max_fire_prot_lvl * 0.15)
 		end
 	end
 
-	if not storage.burn_time or burn_time >= storage.burn_time then
+	local storage = mcl_burning.get_storage(obj)
-		if obj:is_player() then
+	if storage.burn_time then
-			mcl_burning.update_hud(obj)
+		if burn_time > storage.burn_time then
+			storage.burn_time = burn_time
+		end
+		return
 	end
 	storage.burn_time = burn_time
 	storage.fire_damage_timer = 0
 
-		local fire_entity = minetest.add_entity(obj:get_pos(), "mcl_burning:fire")
 	local minp, maxp = mcl_burning.get_collisionbox(obj, false, storage)
-		local obj_size = obj:get_properties().visual_size
-
-		local vertical_grow_factor = 1.2
-		local horizontal_grow_factor = 1.1
-		local grow_vector = vector.new(horizontal_grow_factor, vertical_grow_factor, horizontal_grow_factor)
-
 	local size = vector.subtract(maxp, minp)
-		size = vector.multiply(size, grow_vector)
+	size = vector.multiply(size, vector.new(1.1, 1.2, 1.1))
-		size = vector.divide(size, obj_size)
+	size = vector.divide(size, obj:get_properties().visual_size)
-		local offset = vector.new(0, size.y * 10 / 2, 0)
 
+	local fire_entity = minetest.add_entity(obj:get_pos(), "mcl_burning:fire")
 	fire_entity:set_properties({visual_size = size})
-		fire_entity:set_attach(obj, "", offset, {x = 0, y = 0, z = 0})
+	fire_entity:set_attach(obj, "", vector.new(0, size.y * 5, 0), vector.new(0, 0, 0))
-		local fire_luaentity = fire_entity:get_luaentity()
 
+	if obj:is_player() then
+		mcl_burning.update_hud(obj)
+	end
+
+	--  FIXME: does this code make sense?  It removes attached fire luaentities from
+	--  another object that happen to be at the same position.
+	local fire_luaentity = fire_entity:get_luaentity()
 	for _, other in pairs(minetest.get_objects_inside_radius(fire_entity:get_pos(), 0)) do
 		local other_luaentity = other:get_luaentity()
 		if other_luaentity and other_luaentity.name == "mcl_burning:fire" and other_luaentity ~= fire_luaentity then
@@ -122,7 +134,6 @@ function mcl_burning.set_on_fire(obj, burn_time)
 			break
 		end
 	end
-	end
 end
 
 function mcl_burning.extinguish(obj)