Add MV refining search in warped motion experiment

Implemented a MV refining search after the warped motion parameters were
found. Only 4 or 8 positions were checked so there was almost no impact
on encoder speed.

Borg test result:
            avg_psnr     ovr_psnr    ssim
cam_lowres: -0.543%      -0.574%     -0.670%
lowres    : -0.222%      -0.230%     -0.285%

Change-Id: Ic2f6c1fe548b089d50e9c33bb365e6b128aabc93