This 4-point Type-IV asymmetric DST uses the same computation graph as
 the 4-point Type-IV DST.
This change improves the accuracy of the 8-point Type-II DCT:

Old MSE: 1.8927096972341813413041010372151e-06
New MSE: 1.7946367518072710517065436117146e-06

subset-1:

new_dst4@2017-12-04T06:31:41.096Z -> new_dst4a@2017-12-04T06:32:22.698Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0143 |  0.0410 | -0.2166 |  -0.0556 | -0.0379 | -0.0461 |    -0.0002

Change-Id: Ifde11fca987220130c1657306b0df34ec2f3fe25