avoid direct mutation of Item->count field, use Item->pop() instead

I think this change was already applied on the master branch, but I don't remember for sure.

src/pocketmine/item/PaintingItem.php

@@ -96,7 +96,7 @@ class PaintingItem extends Item{
 		$entity = Entity::createEntity("Painting", $blockReplace->getLevel(), $nbt);
 
 		if($entity instanceof Entity){
-			--$this->count;
+			$this->pop();
 			$entity->spawnToAll();
 
 			$player->getLevel()->broadcastLevelEvent($blockReplace->add(0.5, 0.5, 0.5), LevelEventPacket::EVENT_SOUND_ITEMFRAME_PLACE); //item frame and painting have the same sound

src/pocketmine/item/ProjectileItem.php

@@ -54,7 +54,7 @@ abstract class ProjectileItem extends Item{
 			$projectile->setMotion($projectile->getMotion()->multiply($this->getThrowForce()));
 		}
 
-		$this->count--;
+		$this->pop();
 
 		if($projectile instanceof Projectile){
 			$projectileEv = new ProjectileLaunchEvent($projectile);

src/pocketmine/item/SpawnEgg.php

@@ -44,7 +44,7 @@ class SpawnEgg extends Item{
 		$entity = Entity::createEntity($this->meta, $player->getLevel(), $nbt);
 
 		if($entity instanceof Entity){
-			--$this->count;
+			$this->pop();
 			$entity->spawnToAll();
 			return true;
 		}