This ensures we operate at the same precision that Daala uses, which matters
when activity masking is enabled, because of the gain companding.

Metrics from Patchset 4 (which had slightly incorrect rounding):

With activity masking (5 frames only):
av1_pvq_AM_ref_5f@2017-02-07T03:37:53.702Z -> av1_pvq_AM_derf_fix2_coeff_scaling_5f@2017-02-07T00:12:24.427Z

    PSNR |  PSNR Cb |  PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
  0.6653 | -12.3177 | -12.1858 |   0.3350 | 4.1013 |  2.0964 |    -4.0539

In particular for Netflix_Crosswalk_1920x1080_60fps_8bit_420_60f.y4m
 -5.0589 | -22.3077 | -21.2188 |  -7.0389 | -3.3715 |-5.7794 |   -13.1891

I.e., it fixes the large regression with AM on this sequence, and
 substantially improves chroma (at a lesser cost to other metrics).

Without activity masking (5 frames only):
av1_pvq_ref_5f@2017-02-07T03:52:51.279Z -> av1_pvq_derf_fix2_coeff_scaling_5f@2017-02-07T00:12:48.873Z

    PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
  0.0989 | -0.0322 | -0.0464 |   0.1883 | 0.0795 |  0.0579 |     0.0923

Change-Id: I46b808b7c8e4733465f8bebc8336dfd5b75783ec