Do joint motion search iteratively

Allow motion search multiple times iteratively, and break out
the loop if this search couldn't find better motion vectors.
Limit the maximum number of search to 2.

Tests results:
1. stdhd set: 0.311%(overall psnr); 0.346%(ssim).
positive gain on 10 out of 16 clips(best: 2.746% on sunflower;
worst: -0.434% on old_town_cross).
2. derf set: 0.016%(overall psnr); 0.062%(ssim).
positive gain on half of the clips(best: 0.499% on bowing;
worst: -0.387 on city).

Change-Id: Ibf0a51776d4caf7707be0586346db08128117559