Use saturation addition to do rounding for avx2 IDCT

- Found this bug when increasing unit test number to 10000.
- Unit test is therefore also updated.

Change-Id: I938e96f6ebd35ae1bd8affebf8665e1da49a324b
diff --git a/test/partial_idct_test.cc b/test/partial_idct_test.cc
index 872f33a..0697791 100644
--- a/test/partial_idct_test.cc
+++ b/test/partial_idct_test.cc
@@ -52,7 +52,7 @@
                         TX_SIZE, int, int, int>
     PartialInvTxfmParam;
 const int kMaxNumCoeffs = 1024;
-const int kCountTestBlock = 1000;
+const int kCountTestBlock = 10000;
 
 class PartialIDctTest : public ::testing::TestWithParam<PartialInvTxfmParam> {
  public: