This causes rANS to operate at the same precision as the Daala EC.

aom/master stats: rans10uabs8lbase12 → rans15uabs8lbase15

objective-1-fast
PSNR YCbCr:      0.01%      0.01%      0.01%
   PSNRHVS:      0.01%
      SSIM:      0.01%
    MSSSIM:      0.01%
 CIEDE2000:      0.01%

subset1
PSNR YCbCr:     -0.01%     -0.00%     -0.00%
   PSNRHVS:     -0.01%
      SSIM:     -0.01%
    MSSSIM:     -0.01%
 CIEDE2000:     -0.01%

(cherry picked from aom/master commit ddbc2e2a)

Change-Id: I6ef0a4f6198784b3712a61af9f105d560a22eaea