Fix vectorized subsampling

src/ImageSharp/Formats/Jpeg/Components/Block8x8F.cs

@@ -494,12 +494,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
             var f2 = Vector256.Create(2f);
             var f025 = Vector256.Create(0.25f);
             Vector256<int> switchInnerDoubleWords = Unsafe.As<byte, Vector256<int>>(ref MemoryMarshal.GetReference(SimdUtils.HwIntrinsics.PermuteMaskSwitchInnerDWords8x32));
-
-            ref Vector256<float> in1 = ref Unsafe.As<Block8x8F, Vector256<float>>(ref MemoryMarshal.GetReference(source));
-            ref Vector256<float> in2 = ref Unsafe.As<Block8x8F, Vector256<float>>(ref Unsafe.Add(ref MemoryMarshal.GetReference(source), 1));
             ref Vector256<float> destRef = ref Unsafe.As<Block8x8F, Vector256<float>>(ref destination);
 
-            for (int i = 0; i < 8; i++)
+            for (int i = 0; i < 2; i++)
+            {
+                ref Vector256<float> in1 = ref Unsafe.As<Block8x8F, Vector256<float>>(ref Unsafe.Add(ref MemoryMarshal.GetReference(source), 2 * i));
+                ref Vector256<float> in2 = ref Unsafe.As<Block8x8F, Vector256<float>>(ref Unsafe.Add(ref MemoryMarshal.GetReference(source), (2 * i) + 1));
+
+                for (int j = 0; j < 4; j++)
                 {
                     Vector256<float> a = in1;
                     Vector256<float> b = Unsafe.Add(ref in1, 1);
@@ -521,6 +523,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
                     in1 = ref Unsafe.Add(ref in1, 2);
                     in2 = ref Unsafe.Add(ref in2, 2);
                 }
+            }
 #endif
         }