Add option to disable relative projectile velocity

Allows server owners to use 1.8 (and prior)'s projectile behavior
(ignored shooter's velocity when calculating projectile's velocity).

This patch adds an option "disable relative projectile velocity", which, when
enabled, will cause projectiles to ignore the shooter's current velocity,
like they did in Minecraft 1.8 and prior.
If a player is falling, for example, their shooting range will be drastically
reduced, as a downwards velocity is applied to the projectile. This prevents
players from saving themselves from falling off floating islands, for example,
as a thrown ender pearl will not make it back to the island, while it would
have in 1.8.

While this could easily be done with plugins, too, there are multiple problems:
1) If multiple plugins cancel the velocity by subtracting the shooter's velocity
   from the projectile's velocity, the projectile's velocity would be different.
   As there's no way to detect whether the projectile's velocity has already been
   adjusted to ignore the player's velocity, plugins can't not do it if it's not
   necessary.
2) I've noticed some inconsistencies, e.g. weird velocity when shooting while
   using an elytra. Checking for those inconsistencies is possible, but not as
   efficient as just not applying the velocity in the first place.
3) Solutions for 1) and especially 2) might not be future-proof, while this
   server-internal fix makes this change future-proof.

Spigot-Server-Patches/0442-Configurable-projectile-relative-velocity.patch

@@ -0,0 +1,83 @@
+From 811b4e42f5a03c6fa2e4c1865d91946775cf6118 Mon Sep 17 00:00:00 2001
+From: Lucavon <lucavonlp@gmail.com>
+Date: Sun, 26 May 2019 15:36:04 +0200
+Subject: [PATCH] Configurable projectile relative velocity
+
+This patch adds an option "disable relative projectile velocity", which, when
+nabled, will cause projectiles to ignore the shooter's current velocity,
+like they did in Minecraft 1.8 and prior.
+If a player is falling, for example, their shooting range will be drastically
+reduced, as a downwards velocity is applied to the projectile. This prevents
+players from saving themselves from falling off floating islands, for example,
+as a thrown ender pearl will not make it back to the island, while it would
+have in 1.8.
+
+While this could easily be done with plugins, too, there are multiple problems:
+P1) If multiple plugins cancel the velocity by subtracting the shooter's velocity
+from the projectile's velocity, the projectile's velocity would be different.
+As there's no way to detect whether the projectile's velocity has already been
+adjusted to ignore the player's velocity, plugins can't not do it if it's not
+necessary.
+P2) I've noticed some inconsistencies, e.g. weird velocity when shooting while
+using an elytra. Checking for those inconsistencies is possible, but not as
+efficient as just not applying the velocity in the first place.
+P3) Solutions for 1) and especially 2) might not be future-proof, while this
+server-internal fix makes this change future-proof.
+
+diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
+index bfd690ecc..b8131abae 100644
+--- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
++++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
+@@ -474,6 +474,11 @@ public class PaperWorldConfig {
+         log("Disable Unloaded Chunk Enderpearl Exploit: " + (disableEnderpearlExploit ? "enabled" : "disabled"));
+     }
+
++    public boolean disableRelativeProjectileVelocity;
++    private void disableRelativeProjectileVelocity() {
++        disableRelativeProjectileVelocity = getBoolean("game-mechanics.disable-relative-projectile-velocity", false);
++    }
++
+     public boolean villagesLoadChunks = false;
+     private void villagesLoadChunks() {
+         villagesLoadChunks = getBoolean("game-mechanics.villages-load-chunks", false);
+diff --git a/src/main/java/net/minecraft/server/EntityArrow.java b/src/main/java/net/minecraft/server/EntityArrow.java
+index 6aa518f17..736537b45 100644
+--- a/src/main/java/net/minecraft/server/EntityArrow.java
++++ b/src/main/java/net/minecraft/server/EntityArrow.java
+@@ -80,12 +80,13 @@ public abstract class EntityArrow extends Entity implements IProjectile {
+         float f7 = MathHelper.cos(f1 * 0.017453292F) * MathHelper.cos(f * 0.017453292F);
+
+         this.shoot((double) f5, (double) f6, (double) f7, f3, f4);
++        if (!entity.world.paperConfig.disableRelativeProjectileVelocity) { // Paper - allow disabling relative velocity
+         this.motX += entity.motX;
+         this.motZ += entity.motZ;
+         if (!entity.onGround) {
+             this.motY += entity.motY;
+         }
+-
++        } // Paper - allow disabling relative velocity
+     }
+
+     public void shoot(double d0, double d1, double d2, float f, float f1) {
+diff --git a/src/main/java/net/minecraft/server/EntityProjectile.java b/src/main/java/net/minecraft/server/EntityProjectile.java
+index 60ab1c751..5dae5d604 100644
+--- a/src/main/java/net/minecraft/server/EntityProjectile.java
++++ b/src/main/java/net/minecraft/server/EntityProjectile.java
+@@ -44,12 +44,13 @@ public abstract class EntityProjectile extends Entity implements IProjectile {
+         float f7 = MathHelper.cos(f1 * 0.017453292F) * MathHelper.cos(f * 0.017453292F);
+
+         this.shoot((double) f5, (double) f6, (double) f7, f3, f4);
++        if (!entity.world.paperConfig.disableRelativeProjectileVelocity) { // Paper - allow disabling relative velocity
+         this.motX += entity.motX;
+         this.motZ += entity.motZ;
+         if (!entity.onGround) {
+             this.motY += entity.motY;
+         }
+-
++        } // Paper - allow disabling relative velocity
+     }
+
+     public void shoot(double d0, double d1, double d2, float f, float f1) {
+-- 
+2.22.0
+