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/FileFormats/ShpD2Reader.cs

@@ -86,8 +86,8 @@ namespace OpenRA.FileFormats
 
 	public class ShpD2Reader : ISpriteSource
 	{
-		List<Frame> headers = new List<Frame>();
-		public IEnumerable<ISpriteFrame> Frames { get { return headers.Cast<ISpriteFrame>(); } }
+		readonly List<ISpriteFrame> frames = new List<ISpriteFrame>();
+		public IEnumerable<ISpriteFrame> Frames { get { return frames; } }
 		public bool CacheWhenLoadingTileset { get { return false; } }
 
 		public ShpD2Reader(Stream s)
@@ -108,7 +108,7 @@ namespace OpenRA.FileFormats
 			for (var i = 0; i < imageCount; i++)
 			{
 				s.Position = offsets[i];
-				headers.Add(new Frame(s));
+				frames.Add(new Frame(s));
 			}
 		}
 	}