Small changes to the default zero bin and rounding tables.
Though the tables are currently the same for the Y1 and Y2 cases
I have left them as separate tables in case we want to tune this later.

There is now some adjustment of the zbin based on the prediction mode.
Previously this was restricted to an adjustment for gf/arf 0,0 MV.

The exact quantizer now marginal outperforms and is the default.

The overall average gain is about 0.5%

Change-Id: I5e4353f3d5326dde4e86823684b236a1e9ea7f47