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

OpenRA.Game/Graphics/SpriteLoader.cs

@@ -77,14 +77,23 @@ namespace OpenRA.Graphics
 		public static ISpriteFrame[] GetFrames(IReadOnlyFileSystem fileSystem, string filename, ISpriteLoader[] loaders)
 		{
 			using (var stream = fileSystem.Open(filename))
+			{
+				var spriteFrames = GetFrames(stream, loaders);
+				if (spriteFrames == null)
+					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;
 
-				throw new InvalidDataException(filename + " is not a valid sprite file!");
-			}
+			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;