Fix strange d2k loading slowness

There is a strange issue that appears* when Theater calls
ISpriteFrame.Frames on the R8Reader. The R8Reader uses
IEnumerable.Cast<> which behaves slower and slower, which
makes map loading become 10+ times slower.

The changes here simply avoid the casting.

[*] This happens at least on Linux x86_64 with Mono 3.2.8.

See https://bugzilla.xamarin.com/show_bug.cgi?id=19668

OpenRA.Game/Graphics/SpriteSource.cs

@@ -26,6 +26,7 @@ namespace OpenRA.Graphics
 
 	public interface ISpriteSource
 	{
+		// TODO: Change this to IReadOnlyList so users don't need to call .ToArray()
 		IEnumerable<ISpriteFrame> Frames { get; }
 		bool CacheWhenLoadingTileset { get; }
 	}