Adjust chroma offsets for frame-mean spatiotemporal scale
Adjust chroma offsets for frame-mean spatiotemporal scale

This removes a long-standing implicit boost of chroma quality for
tune=Psychovisual. Which looks a lot like stealing chroma bits to
boost luma, so it is instructive to compare against a baseline of
tune=Psnr.

master-537b1b91-psnr -> master-537b1b91-psy:

PSNR Y |  PSNR Cb |  PSNR Cr | CIEDE2000 |     SSIM |  MS-SSIM
4.0327 | -13.3105 | -13.5377 |   -7.3436 | -11.4949 | -12.0294

PSNR-HVS Y | PSNR-HVS Cb | PSNR-HVS Cr | PSNR-HVS |   VMAF | VMAF-NEG
   -0.6906 |    -16.1168 |    -16.4373 |  -1.1119 | 2.9790 |   3.4978

master-537b1b91-psnr -> steal-chroma-bits-psy:

PSNR Y | PSNR Cb | PSNR Cr | CIEDE2000 |     SSIM |  MS-SSIM
1.9375 | -0.5029 | -1.2879 |   -5.4012 | -13.2846 | -13.7977

PSNR-HVS Y | PSNR-HVS Cb | PSNR-HVS Cr | PSNR-HVS |   VMAF | VMAF-NEG
   -2.7330 |     -3.4827 |     -4.4729 |  -2.5604 | 1.1417 |   1.5887

Note how the PSNR-HVS differences are more consistent between planes.

master-537b1b91-psy -> steal-chroma-bits-psy:

 PSNR Y | PSNR Cb | PSNR Cr | CIEDE2000 |    SSIM | MS-SSIM
-1.9474 | 14.9974 | 14.2232 |    2.2253 | -1.9988 | -1.9685

PSNR-HVS Y | PSNR-HVS Cb | PSNR-HVS Cr | PSNR-HVS |    VMAF | VMAF-NEG
   -2.0192 |     15.1318 |     14.1927 |  -1.3939 | -1.7462 |  -1.8068

xiph/rav1e,xiph/rav1e