Retune constants for ssim_boost

[barrbrain]

There are two target properties:

• When svar == dvar, ssim_boost ~= k / cbrt(svar)
• The constant k is chosen such that mean(log(ssim_boost)) ~= 0

The value of k derived from subset3 was very near cbrt(2).

AWCY results for objective-1-fast at default speed:

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
0.0814	1.3359	1.4880	0.3151	-0.6091	-0.6798	-0.1721	1.2087	1.4406	-0.0515	-0.0085	0.0551

AWCY results for subset1 at default speed:

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
1.2789	N/A	0.9796	0.5621	-1.2553	-0.8473	0.2066	N/A	N/A	0.2358	1.5926	1.5765

[shssoichiro]

This looks cool! My only concern is that it increases the filesize by about 10% on average. Is there anything we can do about that without negating the quality improvements?

[coveralls]

Coverage increased (+0.2%) to 85.269% when pulling 4f61531 on barrbrain:ssim-boost-icbrt into cd25677 on xiph:master.

[barrbrain]

AWCY results for subset1 in combination with segmentation changes:

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
2.1613	1.5164	1.9015	0.4371	-2.8940	-2.2272	0.5407	0.5901	0.5884	0.6090	1.9500	2.0070

For still frames, there is no net change in average file size compared to the baseline before segmentation rework.
We still need some work on tuning the temporal part of RDO.