Change temporal filter's search_method on speed 1

Change temporal filter's search_method on speed 1

This commit introduces a new speed feature that determines the
SEARCH_METHOD used by temporal filter when doing 16x16 block on
full_pixel_motion_search. On speed 0, the most exhaustive method MESH is
used. On speed 1 and above, a faster method NSTEP is used.

Performance:
        | AVG_PSNR | AVG_SPDUP | AVG_SPDUP:AVG_PSNR
 MISRES |  0.007%  |   2.818%  |        402:1
  HDRES |  0.004%  |   4.897%  |       1224:1

In the case of midres, there is a small quality gain of -0.021% on
OVR_PSNR.

Performance measurement is done on speed 1 with auto_alt_ref=1.
Quality is measured on full midres set over 60 frames. Speed is measured
on 5 midres clips over 4 bitrates over 30 frames.

STATS_CHANGED

Change-Id: Ic1879d2237f8734529e194767a6cf5e43e20b47b