Modify HEX search

Changed 8-neighbor searching to 4-neighour searching, and continued
searching until the center point is the best match.

Test on test set showed 1.3% encoding speed improvement as well as
0.1% PSNR and SSIM improvement at speed=-5 (rt mode).

Will continue to improve it.

Change-Id: If4993b1907dd742b906fd3f86fee77cc5932ee9a