Fix mistakes in final touches

Author: tkp1n

src/ImageSharp/Formats/Jpeg/Components/Encoder/RgbToYCbCrConverterVectorized.cs

@@ -169,11 +169,11 @@ private static void ConvertInternal(ReadOnlySpan<Rgb24> rgbSpan, ref Block8x8F y
 
                     Vector256<float> tmpCb = Avx.Permute2x128(cb0, cb1, 0b0010_0001);
                     Unsafe.Add(ref destCbRef, j) = Avx.Blend(cb0, tmpCb, 0b1111_0000);
-                    Unsafe.Add(ref destCbRef, j + 2) = Avx.Blend(cb0, tmpCb, 0b0000_1111);
+                    Unsafe.Add(ref destCbRef, j + 2) = Avx.Blend(cb1, tmpCb, 0b0000_1111);
 
                     Vector256<float> tmpCr = Avx.Permute2x128(cr0, cr1, 0b0010_0001);
                     Unsafe.Add(ref destCrRef, j) = Avx.Blend(cr0, tmpCr, 0b1111_0000);
-                    Unsafe.Add(ref destCrRef, j + 2) = Avx.Blend(cr0, tmpCr, 0b0000_1111);
+                    Unsafe.Add(ref destCrRef, j + 2) = Avx.Blend(cr1, tmpCr, 0b0000_1111);
                 }
             }
         }

src/ImageSharp/Formats/Jpeg/Components/Encoder/YCbCrForwardConverter{TPixel}.cs

@@ -47,7 +47,7 @@ internal ref struct YCbCrForwardConverter<TPixel>
         public static YCbCrForwardConverter<TPixel> Create()
         {
             var result = default(YCbCrForwardConverter<TPixel>);
-            if (RgbToYCbCrConverterVectorized.IsSupported)
+            if (!RgbToYCbCrConverterVectorized.IsSupported)
             {
                 // Avoid creating lookup tables, when vectorized converter is supported
                 result.colorTables = RgbToYCbCrConverterLut.Create();