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/0405-Configurable-projectile-relative-velocity.patch

@@ -0,0 +1,69 @@
+From d8c12019140334bf58354a929f9681fd457f6c66 Mon Sep 17 00:00:00 2001
+From: Lucavon <lucavonlp@gmail.com>
+Date: Tue, 23 Jul 2019 20:29:20 -0500
+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 ff520d9e86..318a470eea 100644
+--- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
++++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
+@@ -557,4 +557,9 @@ public class PaperWorldConfig {
+         }
+         log("Anti-Xray: " + (antiXray ? "enabled" : "disabled") + " / Engine Mode: " + engineMode.getDescription() + " / Chunk Edge Mode: " + chunkEdgeMode.getDescription() + " / Up to " + ((maxChunkSectionIndex + 1) * 16) + " blocks / Update Radius: " + updateRadius);
+     }
++
++    public boolean disableRelativeProjectileVelocity;
++    private void disableRelativeProjectileVelocity() {
++        disableRelativeProjectileVelocity = getBoolean("game-mechanics.disable-relative-projectile-velocity", false);
++    }
+ }
+diff --git a/src/main/java/net/minecraft/server/EntityArrow.java b/src/main/java/net/minecraft/server/EntityArrow.java
+index af0f5cb00f..25ddaae482 100644
+--- a/src/main/java/net/minecraft/server/EntityArrow.java
++++ b/src/main/java/net/minecraft/server/EntityArrow.java
+@@ -85,7 +85,7 @@ 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);
+-        this.setMot(this.getMot().add(entity.getMot().x, entity.onGround ? 0.0D : entity.getMot().y, entity.getMot().z));
++        if (!entity.world.paperConfig.disableRelativeProjectileVelocity) this.setMot(this.getMot().add(entity.getMot().x, entity.onGround ? 0.0D : entity.getMot().y, entity.getMot().z)); // Paper - allow disabling relative velocity
+     }
+ 
+     @Override
+diff --git a/src/main/java/net/minecraft/server/EntityProjectile.java b/src/main/java/net/minecraft/server/EntityProjectile.java
+index 18d28a151a..bd4ca73f6d 100644
+--- a/src/main/java/net/minecraft/server/EntityProjectile.java
++++ b/src/main/java/net/minecraft/server/EntityProjectile.java
+@@ -43,7 +43,7 @@ public abstract class EntityProjectile extends Entity implements IProjectile {
+         this.shoot((double) f5, (double) f6, (double) f7, f3, f4);
+         Vec3D vec3d = entity.getMot();
+ 
+-        this.setMot(this.getMot().add(vec3d.x, entity.onGround ? 0.0D : vec3d.y, vec3d.z));
++        if (!entity.world.paperConfig.disableRelativeProjectileVelocity) this.setMot(this.getMot().add(vec3d.x, entity.onGround ? 0.0D : vec3d.y, vec3d.z)); // Paper - allow disabling relative velocity
+     }
+ 
+     @Override
+-- 
+2.22.0
+