This change improves the accuracy of the 4-point Type-II DCT:

Old MSE: 6.2711279572488185887270981198199e-08
New MSE: 6.0281623825882593130347914239103e-08

It also reduces a multiplication constant so it is less than 1.

subset-1:

daala_tx@2017-12-04T01:58:11.321Z -> new_dst2a@2017-12-04T01:59:12.884Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0274 |  0.0255 |  0.0969 |  -0.0024 | 0.0274 |  0.0027 |     0.0110

Change-Id: I7c9d389af8e98cb39f3bc5923134b5dfe174ba0a