This change slightly improves the 16-point DCT round trip accuracy due
 to changes in the rounding.

subset-1:

new_dst2@2017-12-04T01:59:57.412Z -> new_dst4@2017-12-04T06:31:41.096Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0078 | -0.0001 |  0.0198 |   0.0432 | 0.0408 |  0.0502 |    -0.0057

Change-Id: I75783ace97834af89e70c9ce3002c6f09176e343