change scale down by 4 to use rounding.

source/scale.cc

Index: source/scale.cc
diff --git a/source/scale.cc b/source/scale.cc
index c1cd77c7f110874ae50acf55a27f0d21d1b27096..0dfd99b20606fae45ae2b3bf94309c79af3c42d0 100644
--- a/source/scale.cc
+++ b/source/scale.cc
@@ -182,12 +182,12 @@ static void ScalePlaneDown4(int src_width, int src_height,
     }
   }
 #endif
-#if defined(HAS_SCALEROWDOWN4_SSE2)
-  if (TestCpuFlag(kCpuHasSSE2)) {
+#if defined(HAS_SCALEROWDOWN4_SSSE3)
+  if (TestCpuFlag(kCpuHasSSSE3)) {
     ScaleRowDown4 = filtering ?
-        ScaleRowDown4Box_Any_SSE2 : ScaleRowDown4_Any_SSE2;
+        ScaleRowDown4Box_Any_SSSE3 : ScaleRowDown4_Any_SSSE3;
     if (IS_ALIGNED(dst_width, 8)) {
-      ScaleRowDown4 = filtering ? ScaleRowDown4Box_SSE2 : ScaleRowDown4_SSE2;
+      ScaleRowDown4 = filtering ? ScaleRowDown4Box_SSSE3 : ScaleRowDown4_SSSE3;
     }
   }
 #endif