Fixes robot stacks transferability

Cleans up transferring between stacks and makes robot stacks
transferable with regular ones.

code/game/objects/items/stacks/sheets/glass.dm

@@ -28,6 +28,7 @@
 	icon_state = "sheet-glass"
 	matter = null
 	created_window = /obj/structure/window/basic
+	stacktype = /obj/item/stack/sheet/glass
 
 /obj/item/stack/sheet/glass/attack_self(mob/user as mob)
 	construct_window(user)

code/game/objects/items/stacks/sheets/mineral.dm

@@ -140,6 +140,7 @@ obj/item/stack/sheet/mineral/iron/New()
 	name = "plastic sheets"
 	icon_state = "sheet-plastic"
 	perunit = 2000
+	stacktype = /obj/item/stack/sheet/mineral/plastic
 
 /obj/item/stack/sheet/mineral/gold
 	name = "gold"

code/game/objects/items/stacks/sheets/sheet_types.dm

@@ -92,6 +92,7 @@ var/global/list/datum/stack_recipe/metal_recipes = list ( \
 	icon_state = "sheet-metal"
 	throwforce = 14.0
 	flags = FPRINT | TABLEPASS | CONDUCT
+	stacktype = /obj/item/stack/sheet/metal
 
 /obj/item/stack/sheet/metal/New(var/loc, var/amount=null)
 	recipes = metal_recipes
@@ -152,6 +153,7 @@ var/global/list/datum/stack_recipe/wood_recipes = list ( \
 	desc = "One can only guess that this is a bunch of wood."
 	singular_name = "wood plank"
 	icon_state = "sheet-wood"
+	stacktype = /obj/item/stack/sheet/wood
 
 /obj/item/stack/sheet/wood/New(var/loc, var/amount=null)
 	recipes = wood_recipes