replace missing gui formspec bg variables for chest and furnace

(fixes the background images that went missing a while back)
also, added a corresponding image for the bookshelf

mods/default/furnace.lua

@@ -7,7 +7,7 @@ local function active_formspec(fuel_percent, item_percent)
 	local formspec = 
 		"size[8,8.5]"..
 		default.gui_bg..
-		default.gui_bg_img..
+		default.furnace_bg_img..
 		default.gui_slots..
 		"list[current_name;src;2.75,0.5;1,1;]"..
 		"list[current_name;fuel;2.75,2.5;1,1;]"..
@@ -25,7 +25,7 @@ end
 local inactive_formspec =
 	"size[8,8.5]"..
 	default.gui_bg..
-	default.gui_bg_img..
+	default.furnace_bg_img..
 	default.gui_slots..
 	"list[current_name;src;2.75,0.5;1,1;]"..
 	"list[current_name;fuel;2.75,2.5;1,1;]"..

mods/default/init.lua

@@ -13,6 +13,10 @@ default.gui_bg = "bgcolor[#08080899;false]"
 default.gui_bg_img = "background[5,5;1,1;gui_formbg.png;true]"
 default.gui_slots = "listcolors[#00000069;#5A5A5A;#141318;#30434C;#FFF]"
 
+default.chest_bg_img = "background[5,5;1,1;bg_chest.jpg;true]"
+default.furnace_bg_img = "background[5,5;1,1;bg_furnace.jpg;true]"
+default.bookshelf_bg_img = "background[5,5;1,1;bg_bookshelf.jpg;true]"
+
 function default.get_hotbar_bg(x,y)
 	local out = ""
 	for i=0,7,1 do

mods/default/nodes.lua

@@ -1207,7 +1207,7 @@ minetest.register_node("default:torch", {
 local chest_formspec =
 	"size[8,9]"..
 	default.gui_bg..
-	default.gui_bg_img..
+	default.chest_bg_img..
 	default.gui_slots..
 	"list[current_name;main;0,0.3;8,4;]"..
 	"list[current_player;main;0,4.85;8,1;]"..
@@ -1219,7 +1219,7 @@ local function get_locked_chest_formspec(pos)
 	local formspec =
 		"size[8,9]"..
 		default.gui_bg..
-		default.gui_bg_img..
+		default.chest_bg_img..
 		default.gui_slots..
 		"list[nodemeta:".. spos .. ";main;0,0.3;8,4;]"..
 		"list[current_player;main;0,4.85;8,1;]"..
@@ -1347,7 +1347,7 @@ minetest.register_node("default:chest_locked", {
 local bookshelf_formspec =
 	"size[8,7;]"..
 	default.gui_bg..
-	default.gui_bg_img..
+	default.bookshelf_bg_img..
 	default.gui_slots..
 	"list[context;books;0,0.3;8,2;]"..
 	"list[current_player;main;0,2.85;8,1;]"..