Core/Movement: reworked chase movement

* the destination prediction is now much more reliable
* the movement generator will now more frequently check for valid positions to match retail behavior
* the catch-up mechanic will now adapt its speed to the target's one when being close enough to hit it
* temporarily disabled backwards movement when being too close to a target to avoid some edge cases that need additional research
* fixed the pathfinding reaching behavior so that they no longer follow you to unreachable obstacles and return home when they can't reach you anymore

closes #345
closes #260

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -156,7 +156,7 @@ void ScriptedAI::UpdateAI(uint32 /*diff*/)
     DoMeleeAttackIfReady();
 }
 
-void ScriptedAI::DoStartMovement(Unit* victim, float distance, float angle)
+void ScriptedAI::DoStartMovement(Unit* victim, Optional<float> distance /*= std::nullopt*/, Optional<float> angle /*= std::nullopt*/)
 {
     if (victim)
         me->GetMotionMaster()->MoveChase(victim, distance, angle);

src/server/game/AI/ScriptedAI/ScriptedCreature.h

@@ -162,7 +162,7 @@ struct TC_GAME_API ScriptedAI : public CreatureAI
     // *************
 
     //Start movement toward victim
-    void DoStartMovement(Unit* target, float distance = 0.0f, float angle = 0.0f);
+    void DoStartMovement(Unit* target, Optional<float> distance = std::nullopt, Optional<float> angle = std::nullopt);
 
     //Start no movement on victim
     void DoStartNoMovement(Unit* target);