Really fix Alpha conversion

src/ImageSharp/PixelFormats/Alpha8.cs

@@ -174,13 +174,7 @@ namespace SixLabors.ImageSharp.PixelFormats
 
         /// <inheritdoc/>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void ToRgba64(ref Rgba64 dest)
-        {
-            dest.R = 0;
-            dest.G = 0;
-            dest.B = 0;
-            dest.A = this.PackedValue;
-        }
+        public void ToRgba64(ref Rgba64 dest) => dest.PackFromScaledVector4(this.ToScaledVector4());
 
         /// <summary>
         /// Compares an object with the packed vector.