Core/Entities: improve unit on death movement

Handle MoveFall spline manually instead on MotionMaster -> prepares and sends movespline just before Set methods -> they "detect" the fall spline and send the necessary DISABLE related packets
Signal MotionMaster to not reset-initialize static idle movement with the new MOTIONMASTER_FLAG_STATIC_PREVENT_INITIALIZATION
Prevent spline from generating a path, we want a straight line towards the floor

Author: ccrs

src/server/game/Entities/Creature/Creature.cpp

@@ -2006,13 +2006,6 @@ void Creature::setDeathState(DeathState s)
         if (m_formation && m_formation->GetLeader() == this)
             m_formation->FormationReset(true);
 
-        bool needsFalling = (IsFlying() || IsHovering()) && !IsUnderWater();
-        SetHover(false, false);
-        SetDisableGravity(false, false);
-
-        if (needsFalling)
-            GetMotionMaster()->MoveFall();
-
         Unit::setDeathState(CORPSE);
     }
     else if (s == JUST_RESPAWNED)

src/server/game/Entities/Unit/Unit.cpp

@@ -8760,9 +8760,28 @@ void Unit::setDeathState(DeathState s)
         if (!isOnVehicle)
         {
             if (GetMotionMaster()->StopOnDeath())
-                DisableSpline();
+            {
+                if (!HasUnitState(UNIT_STATE_ROOT | UNIT_STATE_STUNNED) && GetTypeId() == TYPEID_UNIT && ToCreature()->IsInAir(*this, GetFloorZ(), false) && !IsUnderWater())
+                {
+                    GetMotionMaster()->AddFlag(MOTIONMASTER_FLAG_STATIC_PREVENT_INITIALIZATION);
+                    SetFall(true);
+                    Movement::MoveSplineInit init(this);
+                    init.MoveTo(GetPositionX(), GetPositionY(), GetFloorZ(), false, true);
+                    init.SetFall();
+                    init.Launch();
+                }
+                else
+                {
+                    StopMoving();
+                    DisableSpline();
+                }
+            }
         }
 
+        SetDisableGravity(false);
+        SetCanFly(false);
+        SetHover(false);
+
         // without this when removing IncreaseMaxHealth aura player may stuck with 1 hp
         // do not why since in IncreaseMaxHealth currenthealth is checked
         SetHealth(0);

src/server/game/Movement/MotionMaster.cpp

@@ -563,8 +563,6 @@ bool MotionMaster::StopOnDeath()
         MoveIdle();
     }
 
-    _owner->StopMoving();
-
     return true;
 }