SPIGOT-5562: Entering a boat triggers VehicleExitEvent

This patch prevents entities from riding a vehicle they're already riding on.
When trying to enter a boat the client will send up to two packets for each hand
causing the server to think they should enter the boat twice, despite the first
interaction being sufficient.

nms-patches/Entity.patch

@@ -384,7 +384,13 @@
              this.world.addEntity(entityitem);
              return entityitem;
          }
-@@ -1559,7 +1801,7 @@
+@@ -1554,12 +1796,13 @@
+             return false;
+         } else {
+             if (this.isPassenger()) {
++                if (this.vehicle == entity) return true; // CraftBukkit - SPIGOT-5562: already riding
+                 this.stopRiding();
+             }
  
              this.setPose(EntityPose.STANDING);
              this.vehicle = entity;
@@ -393,7 +399,7 @@
              return true;
          }
      }
-@@ -1584,7 +1826,7 @@
+@@ -1584,7 +1827,7 @@
              Entity entity = this.vehicle;
  
              this.vehicle = null;
@@ -402,7 +408,7 @@
          }
  
      }
-@@ -1593,10 +1835,31 @@
+@@ -1593,10 +1836,31 @@
          this.bb();
      }
  
@@ -435,7 +441,7 @@
              if (!this.world.isClientSide && entity instanceof EntityHuman && !(this.getRidingPassenger() instanceof EntityHuman)) {
                  this.passengers.add(0, entity);
              } else {
-@@ -1604,15 +1867,33 @@
+@@ -1604,15 +1868,33 @@
              }
  
          }
@@ -470,7 +476,7 @@
      }
  
      protected boolean q(Entity entity) {
-@@ -1665,7 +1946,13 @@
+@@ -1665,7 +1947,13 @@
                      this.world.getMethodProfiler().enter("portal");
                      this.portalTicks = i;
                      this.portalCooldown = this.getDefaultPortalCooldown();
@@ -485,7 +491,7 @@
                      this.world.getMethodProfiler().exit();
                  }
  
-@@ -1765,6 +2052,13 @@
+@@ -1765,6 +2053,13 @@
      }
  
      public void setSwimming(boolean flag) {
@@ -499,7 +505,7 @@
          this.setFlag(4, flag);
      }
  
-@@ -1825,16 +2119,56 @@
+@@ -1825,16 +2120,56 @@
      }
  
      public void setAirTicks(int i) {
@@ -559,7 +565,7 @@
      }
  
      public void k(boolean flag) {
-@@ -1986,18 +2320,45 @@
+@@ -1986,18 +2321,45 @@
  
      @Nullable
      public Entity a(WorldServer worldserver) {
@@ -609,7 +615,7 @@
              } else {
                  double d0 = this.locX();
                  double d1 = this.locZ();
-@@ -2023,7 +2384,16 @@
+@@ -2023,7 +2385,16 @@
                  Vec3D vec3d1 = this.getPortalOffset();
  
                  blockposition = new BlockPosition(d0, this.locY(), d1);
@@ -627,7 +633,7 @@
  
                  if (shapedetector_shape == null) {
                      return null;
-@@ -2033,6 +2403,11 @@
+@@ -2033,6 +2404,11 @@
                  vec3d = shapedetector_shape.velocity;
                  f = (float) shapedetector_shape.yaw;
              }
@@ -639,7 +645,7 @@
  
              this.world.getMethodProfiler().exitEnter("reloading");
              Entity entity = this.getEntityType().a((World) worldserver);
-@@ -2043,8 +2418,16 @@
+@@ -2043,8 +2419,16 @@
                  entity.setMot(vec3d);
                  worldserver.addEntityTeleport(entity);
                  if (worldserver.getDimensionKey() == World.THE_END) {
@@ -657,7 +663,7 @@
              }
  
              this.bJ();
-@@ -2253,7 +2636,26 @@
+@@ -2253,7 +2637,26 @@
      }
  
      public void a(AxisAlignedBB axisalignedbb) {