Expand the range of alpha to [-2, 2] in Q3.
Jointly signal the signs, including zeros.
Use the signs to give context for each quadrant
and half-axis. The (0, 0) point is excluded.
Symmetry in alpha_u == alpha_v yields 6 contexts.

Results on Subset1 (Compared to 9136ab7d

 with CFL enabled)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0792 | -0.7535 | -0.7574 |  -0.0639 | -0.0843 | -0.0665 |    -0.3324

Change-Id: I250369692e92a91d9c8d174a203d441217d15063
Signed-off-by: David Michael Barr <b@rr-dav.id.au>