Adds code to return an integer cost list for NSTEP search. Then
uses it for pruned subpel search in speed 3.

derf: -0.06%
Speed on mobcal 720p increaes from 10.28 fps to 10.65 fps.
[Subject to further testing].

Change-Id: Ib591382d25b2c11bcaba9d3a27a93a9d1ab27a96