Entity: Require declaration of gravity and drag via abstract methods

this guarantees that subclasses will actually declare them.

[bc break]

src/entity/projectile/Arrow.php

@@ -52,9 +52,6 @@ class Arrow extends Projectile{
 	private const TAG_PICKUP = "pickup"; //TAG_Byte
 	public const TAG_CRIT = "crit"; //TAG_Byte
 
-	protected $gravity = 0.05;
-	protected $drag = 0.01;
-
 	/** @var float */
 	protected $damage = 2.0;
 
@@ -77,6 +74,10 @@ class Arrow extends Projectile{
 
 	protected function getInitialSizeInfo() : EntitySizeInfo{ return new EntitySizeInfo(0.25, 0.25); }
 
+	protected function getInitialDragMultiplier() : float{ return 0.01; }
+
+	protected function getInitialGravity() : float{ return 0.05; }
+
 	protected function initEntity(CompoundTag $nbt) : void{
 		parent::initEntity($nbt);
 

src/entity/projectile/ExperienceBottle.php

@@ -32,7 +32,7 @@ use function mt_rand;
 class ExperienceBottle extends Throwable{
 	public static function getNetworkTypeId() : string{ return EntityIds::XP_BOTTLE; }
 
-	protected $gravity = 0.07;
+	protected function getInitialGravity() : float{ return 0.07; }
 
 	public function getResultDamage() : int{
 		return -1;

src/entity/projectile/SplashPotion.php

@@ -52,9 +52,6 @@ class SplashPotion extends Throwable{
 
 	public static function getNetworkTypeId() : string{ return EntityIds::SPLASH_POTION; }
 
-	protected $gravity = 0.05;
-	protected $drag = 0.01;
-
 	/** @var bool */
 	protected $linger = false;
 	protected PotionType $potionType;
@@ -64,6 +61,8 @@ class SplashPotion extends Throwable{
 		parent::__construct($location, $shootingEntity, $nbt);
 	}
 
+	protected function getInitialGravity() : float{ return 0.05; }
+
 	public function saveNBT() : CompoundTag{
 		$nbt = parent::saveNBT();
 		$nbt->setShort("PotionId", PotionTypeIdMap::getInstance()->toId($this->getPotionType()));

src/entity/projectile/Throwable.php

@@ -29,11 +29,12 @@ use pocketmine\math\RayTraceResult;
 
 abstract class Throwable extends Projectile{
 
-	protected $gravity = 0.03;
-	protected $drag = 0.01;
-
 	protected function getInitialSizeInfo() : EntitySizeInfo{ return new EntitySizeInfo(0.25, 0.25); }
 
+	protected function getInitialDragMultiplier() : float{ return 0.01; }
+
+	protected function getInitialGravity() : float{ return 0.03; }
+
 	protected function onHitBlock(Block $blockHit, RayTraceResult $hitResult) : void{
 		parent::onHitBlock($blockHit, $hitResult);
 		$this->flagForDespawn();