New inter mode context.

New inter mode context.

This patch creates a new inter mode contest that avoids
a dependence on the reconstructed motion vectors from
neighboring blocks. This was a change requested by
a hardware vendor to improve decode performance.

As part of this change I have also made some modifications
to stats output code (under a flag) to allow accumulation of
inter mode context flags over multiple clips

Some further changes will be required to accommodate the
deprecation of the split mv mode over the next few days.

Performance as stands is around -0.25% on derf and
std-hd but up on the YT and YT-HD sets. With further tuning
or some adjustment to the context criteria it should be
possible to make this change broadly neutral.

Change-Id: Ia15cb4470969b9e87332a59c546ae0bd40676f6c