Piecewise linear fit without activity masking on subset3 intra
and objective-1-fast inter, by simple linear regression.

In combination with 79c0f32c "Remove DCT from od_compute_dist_8x8",
this gives a even trade between perceptual and non-perceptual metrics.

av1_daala_dist_30f@2017-01-23T21:32:27.902Z
 -> daala_dist_scale10_30f@2017-01-31T21:52:07.635Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
2.1080 |  2.9645 |  3.4697 |  -2.2086 | 0.2541 | -2.5232 |     2.1645

Piecewise linear-quadratic fit with activity masking, by same method.

The total effect of activity masking and daala-dist changes, with PVQ:

av1_pvq_5f@2017-01-31T01:05:24.219Z
 -> av1_float_pvq_dist_scale_AM_5f_Jan31_crfix@2017-02-02T15:14:40.477Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM |  MS SSIM | CIEDE 2000
22.5041 | 42.6349 | 40.8516 | -10.3510 | -6.8030 | -16.9057 |    21.2613

Change-Id: I9b513509a03aa058dc5c1479c01d62c8fc363a34