Core/Creature: implement school immunity for creatures in creature_template

Port From (https://github.com/TrinityCore/TrinityCore/commit/ed3b34e33f019d9200a3d003c467e0ffa351187a)

Source/Game/Entities/Creature/Creature.cs

@@ -420,6 +420,7 @@ namespace Game.Entities
             UpdateMovementFlags();
             LoadCreaturesAddon();
             LoadMechanicTemplateImmunity();
+            LoadSpellTemplateImmunity();
             return true;
         }
 
@@ -1892,6 +1893,27 @@ namespace Game.Entities
             }
         }
 
+        void LoadSpellTemplateImmunity()
+        {
+            // uint32 max used for "spell id", the immunity system will not perform SpellInfo checks against invalid spells
+            // used so we know which immunities were loaded from template
+            uint placeholderSpellId = uint.MaxValue;
+
+            // unapply template immunities (in case we're updating entry)
+            for (var i = SpellSchoolMask.Normal; i <= SpellSchoolMask.Arcane; ++i)
+                ApplySpellImmune(placeholderSpellId, SpellImmunity.School, i, false);
+
+            // don't inherit immunities for hunter pets
+            if (GetOwnerGUID().IsPlayer() && IsHunterPet())
+                return;
+
+            byte mask = GetCreatureTemplate().SpellSchoolImmuneMask;
+            if (mask != 0)
+                for (var i = (int)SpellSchoolMask.Normal; i <= (int)SpellSchoolMask.Arcane; ++i)
+                    if ((mask & (1 << i)) != 0)
+                        ApplySpellImmune(placeholderSpellId, SpellImmunity.School, 1u << i, true);
+        }
+
         public override bool IsImmunedToSpell(SpellInfo spellInfo, Unit caster)
         {
             if (spellInfo == null)

Source/Game/Entities/Creature/CreatureData.cs

@@ -89,6 +89,7 @@ namespace Game.Entities
         public int WidgetSetUnitConditionID;
         public bool RegenHealth;
         public uint MechanicImmuneMask;
+        public byte SpellSchoolImmuneMask;
         public CreatureFlagsExtra FlagsExtra;
         public uint ScriptID;
 

Source/Game/Globals/ObjectManager.cs

@@ -1870,8 +1870,9 @@ namespace Game
             creature.WidgetSetUnitConditionID = fields.Read<int>(72);
             creature.RegenHealth = fields.Read<bool>(73);
             creature.MechanicImmuneMask = fields.Read<uint>(74);
-            creature.FlagsExtra = (CreatureFlagsExtra)fields.Read<uint>(75);
-            creature.ScriptID = GetScriptId(fields.Read<string>(76));
+            creature.SpellSchoolImmuneMask = fields.Read<byte>(75);
+            creature.FlagsExtra = (CreatureFlagsExtra)fields.Read<uint>(76);
+            creature.ScriptID = GetScriptId(fields.Read<string>(77));
 
             _creatureTemplateStorage[entry] = creature;
         }