Using a tighter resize constraint on undershoot seems to help
results (especially SSIM) as significant undershoot on a frame
seems to have more of a damaging impact than overshoot.

This patch has been tuned so that in local testing using the
derf set it is encode speed neutral for speed  setting 2.

Average quality result for speed 2 (psnr,ssim) were  as follows:-

 lowres  0.039,  0.453
 midres  0.249, 0.853
 hdres  0.159, 0.659
 NetFlix -0.241, 0.360

Change-Id: Ie8d3a0d7d6f7ea89d9965d1821be17f8bda85062