libvpx

Reuse inter prediction result in real-time speed 6

In real-time speed 6, no partition search is done. The inter
prediction results got from picking mode can be reused in the
following encoding process. A speed feature reuse_inter_pred_sby
is added to only enable the resue in speed 6.

This patch doesn't change encoding result. RTC set tests showed
that the encoding speed gain is 2% - 5%.

Change-Id: I3884780f64ef95dd8be10562926542528713b92c