[CFL] Move Subsampling in Store

Pixels are subsampled when they are stored in the CfL prediction
buffer. This allows to avoid having two buffers. No impact on the
bitstream.

Results on subset1 (Compared to parent)

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

https://arewecompressedyet.com/?job=cfl-avg-on-fly%402017-09-23T21%3A38%3A04.440Z&job=cfl-Sub-On-Store%402017-09-24T15%3A01%3A41.161Z

Change-Id: If051e34d6d7078d77609542305a2afebef5cb177