Stop if less than half of the iterations give improvement.

Minor metric changes for a 2.5x speed up of the alpha search.

Results on subset1:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0038 |  0.0466 |  0.1388 |  -0.0103 | -0.0312 | -0.0220 |     0.0330

Change-Id: Ic25a995eee500ffc4b80b73635baf0a710954dc0