Add tuned deadzoning with two-level biasing and separate EOB biasing
Add tuned deadzoning with two-level biasing and separate EOB biasing

The idea is based on what the Thor codebase does (which is
undocumented).

See comments for details on how the constants were derived,
and the algorithm in general.

The intent is that this is used as the fast path once we add
trellis quantization.

This particularly helps high motion clips like Red Kayak and
Ducks Take Off:

                                  |    PSNR | PSNR HVS |    SSIM | CIEDE 2000 |  MS SSIM |    VMAF
   ducks_take_off_1080p50_60f.y4m | -3.5880 |  -4.6035 | -3.3884 |    -1.3255 |  -4.1065 | -9.5272
   red_kayak_360p_60f.y4m         |  0.1735 |  -2.2590 | -0.4001 |    -1.7106 |  -1.9463 | -9.1934

AWCY Results:

      PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    VMAF |    SSIM | MS SSIM | CIEDE 2000
   -0.2979 | -3.7618 | -2.8471 |  -0.5794 | -3.3000 | -0.2043 | -0.3993 |    -1.5294

AWCY Link: https://beta.arewecompressedyet.com/?job=master-162ee92ca6a819c19c6053841a4ab754cb735b16&job=deadzoning1%402019-09-02T14%3A31%3A02.785Z

Implements #1338 and #1340.

Signed-off-by: Derek Buitenhuis <aff7d637ede26ea82e5226e01b697996cc52e526@gmail.com>

xiph/rav1e,xiph/rav1e