Fix the --png command trying to use OpenRA's virtual file system

OpenRA.Game/Graphics/SpriteLoader.cs

@@ -78,13 +78,22 @@ namespace OpenRA.Graphics
 		{
 			using (var stream = fileSystem.Open(filename))
 			{
-				ISpriteFrame[] frames;
-				foreach (var loader in loaders)
-					if (loader.TryParseSprite(stream, out frames))
-						return frames;
+				var spriteFrames = GetFrames(stream, loaders);
+				if (spriteFrames == null)
+					throw new InvalidDataException(filename + " is not a valid sprite file!");
 
-				throw new InvalidDataException(filename + " is not a valid sprite file!");
+				return spriteFrames;
 			}
 		}
+
+		public static ISpriteFrame[] GetFrames(Stream stream, ISpriteLoader[] loaders)
+		{
+			ISpriteFrame[] frames;
+			foreach (var loader in loaders)
+				if (loader.TryParseSprite(stream, out frames))
+					return frames;
+
+			return null;
+		}
 	}
 }

OpenRA.Mods.Common/UtilityCommands/ConvertSpriteToPngCommand.cs

@@ -47,7 +47,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 			var palette = new ImmutablePalette(args[2], shadowIndex);
 
-			var frames = SpriteLoader.GetFrames(modData.DefaultFileSystem, src, modData.SpriteLoaders);
+			var frames = SpriteLoader.GetFrames(File.OpenRead(src), modData.SpriteLoaders);
 
 			var usePadding = !args.Contains("--nopadding");
 			var count = 0;