cost_coefs approximation speed up

This patch adds a new speed feature which doesn't do the rather
expensive entropy context lookup or save to the table,  while
doing costing.

The speed up on desktop36p.y4m is around 10% other clips much less.

On the RTC test set this was + 1% in overall datarate.

Change-Id: Ia5144bbf45270671e7be9c8e4055369909e2f738