Add Argb to PackedPixelConverter to allow transforms.

src/ImageSharp/Colors/PackedPixel/PackedPixelConverterHelper.cs

@@ -300,6 +300,7 @@ namespace ImageSharp
         private static bool IsStandardNormalizedType(Type type)
         {
             return type == typeof(Color)
+                || type == typeof(Argb)
                 || type == typeof(Alpha8)
                 || type == typeof(Bgr565)
                 || type == typeof(Bgra4444)