Fix multiple memory leaks and stale pooled allocations

* Fix Vec3/AABB pool reset logic

  * resetPool()/clearPool() now reset poolPointer to 0
  * Prevents endless pool growth, invalid temporary references,
    and increasing memory usage
  * Resolves the leaderboard-related crash mentioned in
    Windows64_Minecraft.cpp

* Properly free old screens in Minecraft::setScreen()

  * Added virtual destructor to Screen
  * Screen destructor now cleans up GuiParticles and Buttons
  * Previous screen is now deleted after screen transitions
  * Screen::init() now clears old buttons/particles before re-init

* Fix GuiParticles memory leak

  * Removed GuiParticle instances are now deleted in tick()
  * Added GuiParticles destructor to free remaining particles

Minecraft.Client/GuiParticles.cpp

@@ -8,6 +8,15 @@ GuiParticles::GuiParticles(Minecraft *mc)
 	this->mc = mc;
 }
 
+GuiParticles::~GuiParticles()
+{
+	for (GuiParticle *gp : particles)
+	{
+		delete gp;
+	}
+	particles.clear();
+}
+
 void GuiParticles::tick()
 {
     for (unsigned int i = 0; i < particles.size(); i++)
@@ -19,6 +28,7 @@ void GuiParticles::tick()
 
         if (gp->removed)
 		{
+            delete gp;
             particles.erase(particles.begin()+i);
 			i--;
         }