The in/out (or zoom metrics) in accumulate_frame_motion_stats()
are in effect a % of the blocks that have a motion vector pointing
either towards or away from the center. As such they are already
normalized in terms of image size and the thresholds against which
these are tested should be image size independent.

In practice a zoom either in or out is an indicator for a shorter group
length so the abs value is more important as a breakout clause.

This patch fixes the threshold test. Clips without noticeable zoom show
no effect but some  with strong zooms such as "station" show a big
gain (5-10%). Average psnr-hvs gain on hdres set was 0.292%

Change-Id: I4f97a72b0e273e4e844ade15285749c32cd81c1c