Allow backward prob update in external mode info coding flow

This commit enables vpxenc to properly count the coded motion
vector related information for backward update. This allows the
coding flow using external mode info to use backward probability
update. In the short test clip, over 10% bit-rate saving is
observed at no distortion change.

Change-Id: Ie27e97114ab91c3d95ba7b5554d617d226db5e20
1 file changed