Added a sample selection process in warped motion.
1. Gather more samples including multiple rows on the top, multiple
columns on the left, and the upper-right block.
2. Sort samples by the MV difference between the neighbour's MV and
the current block's MV. Trim the samples with considerably large MV
difference.

Borg test result:
             avg_psnr ovr_psnr ssim
cam_lowres:  -0.241   -0.243  -0.376
lowres:      -0.104   -0.110  -0.179

The changes are wrapped in WARPED_MOTION_SORT_SAMPLES macro.

Change-Id: I2730bb31a0a3ad28215ccd16fd6da0ea8b2ed404