Porting "Moves machine/computer frames and structures below object layer." and lowering trays layer.

code/__DEFINES/layers.dm

@@ -39,6 +39,7 @@
 #define DOOR_HELPER_LAYER 2.71 //keep this above OPEN_DOOR_LAYER
 #define PROJECTILE_HIT_THRESHHOLD_LAYER 2.75 //projectiles won't hit objects at or below this layer if possible
 #define TABLE_LAYER 2.8
+#define TRAY_LAYER 2.85
 #define BELOW_OBJ_LAYER 2.9
 #define LOW_ITEM_LAYER 2.95
 //#define OBJ_LAYER 3 //For easy recordkeeping; this is a byond define