Change the adjustment range from [50% ... 300%] to [25% ... 100%].
Adjustments above 100% were very rare, and capping the adjustment at
100% adds SIMD optimisation opportunities.  And lowering the other end
to 25% seems to help compresson slightly.

Low latency, used-cpu=0:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0453 | -0.1118 | -0.1127 |  -0.0689 | -0.0429 | -0.0814 |    -0.0762

High latency, used-cpu=0:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
-0.0303 |  0.0583 |  0.1740 |  -0.0440 | 0.0033 | -0.0042 |     0.0040

Change-Id: Id999158330a53e8c3383cd0e53a91c7f59fe062a