Changes ISpriteSource.Frames to be of type IReadOnlyList<ISpriteFrame>.

- Updated implementations to return a ReadOnlyList around an array (to reduce wasted memory from exposing lists or lazy enumerators around lists).
- Protect non-public ISpriteFrame classes by making them inner classes to prevent casting.
- Added an AsReadOnly extension method for lists.

OpenRA.Game/Graphics/SpriteSource.cs

@@ -1,6 +1,6 @@
 #region Copyright & License Information
 /*
- * Copyright 2007-2011 The OpenRA Developers (see AUTHORS)
+ * Copyright 2007-2014 The OpenRA Developers (see AUTHORS)
  * This file is part of OpenRA, which is free software. It is made
  * available to you under the terms of the GNU General Public License
  * as published by the Free Software Foundation. For more information,
@@ -8,7 +8,6 @@
  */
 #endregion
 
-using System.Collections.Generic;
 using System.Drawing;
 using System.IO;
 using OpenRA.FileFormats;
@@ -25,8 +24,7 @@ namespace OpenRA.Graphics
 
 	public interface ISpriteSource
 	{
-		// TODO: Change this to IReadOnlyList so users don't need to call .ToArray()
-		IEnumerable<ISpriteFrame> Frames { get; }
+		IReadOnlyList<ISpriteFrame> Frames { get; }
 		bool CacheWhenLoadingTileset { get; }
 	}