SPIGOT-7947: addPassenger doesn't work if the vehicle is a player

By: md_5 <git@md-5.net>

paper-server/nms-patches/net/minecraft/world/entity/Entity.patch

@@ -620,6 +620,15 @@
                      leashable.setLeashedTo(entityhuman, true);
                  }
  
+@@ -2243,7 +2596,7 @@
+             return false;
+         } else if (!entity.couldAcceptPassenger()) {
+             return false;
+-        } else if (!this.level().isClientSide() && !entity.type.canSerialize()) {
++        } else if (!flag && !this.level().isClientSide() && !entity.type.canSerialize()) { // SPIGOT-7947: Allow force riding all entities
+             return false;
+         } else {
+             for (Entity entity1 = entity; entity1.vehicle != null; entity1 = entity1.vehicle) {
 @@ -2255,6 +2608,27 @@
              if (!flag && (!this.canRide(entity) || !entity.canAddPassenger(this))) {
                  return false;