libvpx

Motion referenced partition search for non-RD coding flow

This commit enables the non-RD mode decision coding flow to
adaptively apply partition search in non-refresh frame, when the
collocated block in previous frame suggests there might be a motion
activity. It refactors the update_state_rt() function to support
buffer swap of mode_info struct, thereby unifying the encoding
stage across various non-RD coding modes.

It provides 5% compression performance gains in speed -6 for rtc
test set, at about 12% speed slow down.

Change-Id: Iefa374aed5a11c4b7ff9a3ed36a98ea8bd184edb