[CFL] Extract Subsampling Functions
[CFL] Extract Subsampling Functions

We extract these functions in order to move them to the cfl_store in a
subsequent commit. This change does not impact the bitstream.

Results on Subset1 (compared to parent with CfL enabled)
  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-inter%402017-09-23T20%3A51%3A01.093Z&job=cfl-extract%402017-09-23T20%3A54%3A15.693Z

Change-Id: I84bfe60e25a4a665f686d6b18789395a50318a9b

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom