Add SSE2 version of SimpleHFilter16i

src/ImageSharp/Formats/Webp/Lossy/LossyUtils.cs

@@ -1303,10 +1303,23 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
 
         public static void SimpleHFilter16i(Span<byte> p, int offset, int stride, int thresh)
         {
-            for (int k = 3; k > 0; k--)
+#if SUPPORTS_RUNTIME_INTRINSICS
+            if (Sse2.IsSupported)
             {
-                offset += 4;
-                SimpleHFilter16(p, offset, stride, thresh);
+                for (int k = 3; k > 0; k--)
+                {
+                    offset += 4;
+                    SimpleHFilter16(p, offset, stride, thresh);
+                }
+            }
+            else
+#endif
+            {
+                for (int k = 3; k > 0; k--)
+                {
+                    offset += 4;
+                    SimpleHFilter16(p, offset, stride, thresh);
+                }
             }
         }