Dual pred flag

Further changes to make experiments with the context
used for coding the dual pred flag easier.

Current best performing method tested on derf is a two
element context based on reference frame. I also tried
various combinations of mode and reference frame as
shown in commented out case using up to 6 contexts.

Derf +0.26 overall psnr +0.15% ssim vs original method.

Change-Id: I64c21ddec0abbb27feaaeaa1da2e9f164ebaca03