Adds common proc to obtain the material of an object

Adds a common get_material() proc to objects, and implements overrides
where applicable

code/game/objects/structures/simple_doors.dm

@@ -44,6 +44,9 @@
 	update_nearby_tiles()
 	..()
 
+/obj/structure/simple_door/get_material()
+	return material
+
 /obj/structure/simple_door/Bumped(atom/user)
 	..()
 	if(!state)

code/game/objects/structures/stool_bed_chair_nest/bed.dm

@@ -33,6 +33,9 @@
 		padding_material = get_material_by_name(new_padding_material)
 	update_icon()
 
+/obj/structure/bed/get_material()
+	return material
+
 // Reuse the cache/code from stools, todo maybe unify.
 /obj/structure/bed/update_icon()
 	// Prep icon.