Adjust motion component of prediction decay.
Adjust motion component of prediction decay.

Adjust the motion decay component to account for image size.
This has very little impact for smaller image sizes.

Average bdrate results for our HD test sets:-

Hdres set:  opsnr +0,92%,  Fast SSIM +1.6%
Netflix hd set:  opsnr + 1.5%, Fast SSIM +3.1%

There are a couple of notable -ve clips such as cyclist and sunflower
which seem to be better with a shorter interval but also a few very big
wins such as Jets >12% psnr 22% Fast SSIM and from the Netflix
Netflix set PierSeaside 9.7% psnr and 18.2% Fast SSIM.

Change-Id: Ie43aaedaa74331ed83d624a13548094ac64fed9e

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