This 2-point Type-IV DST uses the same computation graph as the
 asymmetric 2-point Type-IV DST.
Because this transform is embedded, it may be possible to remove the
 initial averaging step by splitting the 2-point Type-IV DST into
 separate forward and inverse transforms.
This change also reduces two multiplication constants (forward and
 inverse transform) so they are less than 1.

subset-1:

new_dst2a@2017-12-04T01:59:12.884Z -> new_dst2@2017-12-04T01:59:57.412Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0126 |  0.0387 |  0.0441 |   0.0554 | -0.0301 |  0.0034 |    -0.0342

Change-Id: I98568e0c5b97e3a6af27653ddab845ce97d2a53d