Use unmanaged constraint.

src/ImageSharp/Formats/Jpeg/Components/GenericBlock8x8.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
-using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -16,7 +15,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
     /// </summary>
     [StructLayout(LayoutKind.Sequential)]
     internal unsafe partial struct GenericBlock8x8<T>
-        where T : struct
+        where T : unmanaged
     {
         public const int Size = 64;
 

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -6,7 +6,6 @@ using System.Buffers.Binary;
 using System.IO;
 using System.Linq;
 using System.Runtime.CompilerServices;
-using System.Threading.Tasks;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Jpeg.Components;
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;