Shielding Auras (#8760)

Ports Baystation12/Baystation12#19992 and Baystation12/Baystation12#27266.

    Added a personal shield device to the traitor tools uplink.
    Added a radiant shielding aura spell to Battlemage and Cleric.
    Added an exosuit shield drone to the mechfab.

This works, but I'm not entirely happy with the sprites. The personal shield has no on-mob shimmer, it uses a mindbatterer grenade as an icon sprite. The wizard radiant spell uses a pretty big and janky sprite as an on-mob, and the exosuit's shield sprite doesn't follow the dir the mech faces, but that one I can maybe fix on my own. If anyone knows about that last one, lemme know, Bay had code for it that we do not.

code/__defines/_layers.dm

@@ -21,6 +21,7 @@
 #define SCREEN_LAYER 22			//Mob HUD/effects layer
 #define CINEMA_LAYER 23
 
+#define MECH_UNDER_LAYER            4
 #define MECH_BASE_LAYER             4.01
 #define MECH_INTERMEDIATE_LAYER     4.02
 #define MECH_PILOT_LAYER            4.03
@@ -28,4 +29,5 @@
 #define MECH_COCKPIT_LAYER          4.05
 #define MECH_ARM_LAYER              4.06
 #define MECH_DECAL_LAYER            4.07
-#define MECH_GEAR_LAYER             4.08
+#define MECH_GEAR_LAYER             4.08
+#define MECH_ABOVE_LAYER            4.09

code/__defines/mobs.dm

@@ -344,4 +344,11 @@
 // Surgery Stuff
 #define SURGERY_SUCCESS 2 // Proceed with surgery
 #define SURGERY_FAIL 1 // Autofail surgery
-#define SURGERY_IGNORE 0 // Ignore surgery completely and just attack
+#define SURGERY_IGNORE 0 // Ignore surgery completely and just attack
+
+#define AURA_CANCEL 1
+#define AURA_FALSE  2
+#define AURA_TYPE_BULLET "Bullet"
+#define AURA_TYPE_WEAPON "Weapon"
+#define AURA_TYPE_THROWN "Thrown"
+#define AURA_TYPE_LIFE   "Life"