On a new client circuit, send the initial reply ack to let the client know it's live before sending other data.

This means that avatar/appearance data of other avatars and scene objects for a client will be sent after the ack rather than possibly before.
This may stop some avatars appearing grey on login.
This introduces a new OpenSim.Framework.ISceneAgent to accompany the existing OpenSim.Framework.ISceneObject and ISceneEntity
This allows IClientAPI to handle this as it can't reference OpenSim.Region.Framework.Interfaces

OpenSim/Framework/IClientAPI.cs

@@ -702,6 +702,12 @@ namespace OpenSim.Framework
 
         UUID AgentId { get; }
 
+        /// <summary>
+        /// The scene agent for this client.  This will only be set if the client has an agent in a scene (i.e. if it
+        /// is connected).
+        /// </summary>
+        ISceneAgent SceneAgent { get; }
+
         UUID SessionId { get; }
 
         UUID SecureSessionId { get; }

OpenSim/Framework/IScene.cs

@@ -68,12 +68,16 @@ namespace OpenSim.Framework
         event restart OnRestart;
 
         /// <summary>
-        /// Register the new client with the scene.  The client starts off as a child agent - the later agent crossing
-        /// will promote it to a root agent.
+        /// Add a new client and create a presence for it.  All clients except initial login clients will starts off as a child agent
+        /// - the later agent crossing will promote it to a root agent.
         /// </summary>
         /// <param name="client"></param>
         /// <param name="type">The type of agent to add.</param>
-        void AddNewClient(IClientAPI client, PresenceType type);
+        /// <returns>
+        /// The scene agent if the new client was added.
+        /// Null if the required scene agent already existed or no scene agent was added because the required client circuit doesn't exist.
+        /// </returns>
+        ISceneAgent AddNewClient(IClientAPI client, PresenceType type);
 
         /// <summary>
         /// Remove the given client from the scene.

OpenSim/Framework/ISceneAgent.cs

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) Contributors, http://opensimulator.org/
+ * See CONTRIBUTORS.TXT for a full list of copyright holders.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the OpenSimulator Project nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+using System;
+
+namespace OpenSim.Framework
+{
+    /// <summary>
+    /// An agent in the scene.
+    /// </summary>
+    /// <remarks>
+    /// Interface is a work in progress.  Please feel free to add other required properties and methods.
+    /// </remarks>
+    public interface ISceneAgent : ISceneEntity
+    {
+        /// <value>
+        /// The client controlling this presence
+        /// </value>
+        IClientAPI ControllingClient { get; }
+
+        /// <summary>
+        /// What type of presence is this?  User, NPC, etc.
+        /// </summary>
+        PresenceType PresenceType { get; }
+
+        /// <summary>
+        /// Avatar appearance data.
+        /// </summary>
+        /// <remarks>
+        // Because appearance setting is in a module, we actually need
+        // to give it access to our appearance directly, otherwise we
+        // get a synchronization issue.
+        /// </remarks>
+        AvatarAppearance Appearance { get; set; }
+
+        /// <summary>
+        /// Send initial scene data to the client controlling this agent
+        /// </summary>
+        /// <remarks>
+        /// This includes scene object data and the appearance data of other avatars.
+        /// </remarks>
+        void SendInitialDataToMe();
+    }
+}