1. Reduce the worst-case number of samples from 34 to 16.
2. Simplify the trimming strategy to not use multiplication and
division operations.

Overall, this modification reduces the complexity without affecting
the coding gain too much.

Turning on "ext_warped_motion", the Borg test showed:
             avg_psnr ovr_psnr ssim
cam_lowres:  -0.912   -0.920  -1.246
lowres:      -0.229   -0.219  -0.373

Change-Id: Ida21d1d3f22998c851b1270fe61a5e30f9e4ec38