fix(core): Converts boat packets (#416)

- [X] Converts boat packets
- [X] Makes a packet container builder for packet 0xF7
- [X] Generalizes world item packet so it works for mobiles too

Notes:
- This PR doesn't address proper smooth movement for boats.

Projects/UOContent/Multis/Boats/BoatPackets.cs

@@ -0,0 +1,88 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright (C) 2019-2021 - ModernUO Development Team                   *
+ * Email: hi@modernuo.com                                                *
+ * File: BoatPackets.cs                                                  *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System.Buffers;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using Server.Collections;
+using Server.Network;
+
+namespace Server.Multis.Boats
+{
+    public static class BoatPackets
+    {
+        public static void SendMoveBoatHS(this NetState ns, Mobile beholder, BaseBoat boat,
+            Direction d, int speed, List<IEntity> ents, int xOffset, int yOffset)
+        {
+            if (ns?.HighSeas != true)
+            {
+                return;
+            }
+
+            var maxLength = 18 + ents.Count * 10;
+            var writer = new SpanWriter(stackalloc byte[maxLength]);
+            writer.Write((byte)0xF6); // Packet ID
+            writer.Seek(2, SeekOrigin.Current);
+
+            writer.Write(boat.Serial);
+            writer.Write((byte)speed);
+            writer.Write((byte)d);
+            writer.Write((byte)boat.Facing);
+            writer.Write((short)(boat.X + xOffset));
+            writer.Write((short)(boat.Y + yOffset));
+            writer.Write((short)boat.Z);
+            writer.Seek(2, SeekOrigin.Current); // count
+
+            var count = 0;
+
+            foreach (var ent in ents)
+            {
+                if (!beholder.CanSee(ent))
+                {
+                    continue;
+                }
+
+                writer.Write(ent.Serial);
+                writer.Write((short)(ent.X + xOffset));
+                writer.Write((short)(ent.Y + yOffset));
+                writer.Write((short)ent.Z);
+                ++count;
+            }
+
+            writer.Seek(16, SeekOrigin.Begin);
+            writer.Write((short)count);
+            writer.WritePacketLength();
+
+            ns.Send(writer.Span);
+        }
+
+        public static void SendDisplayBoatHS(this NetState ns, Mobile beholder, BaseBoat boat)
+        {
+            var ents = boat.GetMovingEntities();
+
+            ents.AddNotNull(boat.TillerMan);
+            ents.AddNotNull(boat.Hold);
+            ents.AddNotNull(boat.PPlank);
+            ents.AddNotNull(boat.SPlank);
+
+            ents.Add(boat);
+
+            var eable = ents.Where(beholder.CanSee);
+
+            ns.SendBatchEntities(eable, ents.Count);
+        }
+    }
+}