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/Primitives/ReadOnlyList.cs

@@ -21,12 +21,20 @@ namespace OpenRA
 	/// duplicate it but provide a compatible interface that can be replaced
 	/// when we switch to .NET 4.5 or higher.
 	/// </remarks>
-	public interface IReadOnlyList<T> : IEnumerable<T>
+	public interface IReadOnlyList<out T> : IEnumerable<T>
 	{
 		int Count { get; }
 		T this[int index] { get; }
 	}
 
+	public static class ReadOnlyList
+	{
+		public static IReadOnlyList<T> AsReadOnly<T>(this IList<T> list)
+		{
+			return list as IReadOnlyList<T> ?? new ReadOnlyList<T>(list);
+		}
+	}
+
 	/// <summary>
 	/// A minimal read only list for .NET 4 implemented as a wrapper
 	/// around an IList.