Layers are now defines (#17949)

* Layers are now defines

* this looks better

* GAS_phil_LAYER

* no message

* remove the three unneeded defines

* no message

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

@@ -155,7 +155,7 @@ var/global/list/datum/stack_recipe/sinew_recipes = list ( \
 	singular_name = "hide plate"
 	flags = NOBLUDGEON
 	w_class = 3
-	layer = 4
+	layer = MOB_LAYER
 
 /obj/item/stack/sheet/animalhide/ashdrake
 	name = "ash drake hide"
@@ -165,7 +165,7 @@ var/global/list/datum/stack_recipe/sinew_recipes = list ( \
 	singular_name = "drake plate"
 	flags = NOBLUDGEON
 	w_class = 3
-	layer = 4
+	layer = MOB_LAYER
 
 
 //Step one - dehairing.

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

@@ -63,7 +63,7 @@ var/global/list/datum/stack_recipe/sandstone_recipes = list ( \
 	icon_state = "sandbags"
 	singular_name = "sandbag"
 	force = 5
-	layer = 2.95
+	layer = LOW_ITEM_LAYER
 	throwforce = 5
 	w_class = 3
 	throw_speed = 1