Change gameid to prevent overlap in MP with upstream

Games are segregated on version number and gameid.
The gameid consists in 4 bytes characters describing Hellfire, Hellfire
Spawn, Diablo or Diablo Spawn.

To get a segregation between this mod and upstream and keep the internal
segregation between Hellfire, Hellfire Spawn etc.. this patch does a
bitwise OR between the original 4 bytes and "IRON".

Source/diablo.h

@@ -21,7 +21,8 @@ constexpr uint32_t GameIdDiabloFull = LoadBE32("DRTL");
 constexpr uint32_t GameIdDiabloSpawn = LoadBE32("DSHR");
 constexpr uint32_t GameIdHellfireFull = LoadBE32("HRTL");
 constexpr uint32_t GameIdHellfireSpawn = LoadBE32("HSHR");
-#define GAME_ID (gbIsHellfire ? (gbIsSpawn ? GameIdHellfireSpawn : GameIdHellfireFull) : (gbIsSpawn ? GameIdDiabloSpawn : GameIdDiabloFull))
+constexpr uint32_t GameIdIronman = LoadBE32("IRON");
+#define GAME_ID ((gbIsHellfire ? (gbIsSpawn ? GameIdHellfireSpawn : GameIdHellfireFull) : (gbIsSpawn ? GameIdDiabloSpawn : GameIdDiabloFull)) | GameIdIronman)
 
 #define NUMLEVELS 25