libvpx

Re-arrange inter mode search order in RTC coding flow

This commit makes the ZEROMV mode first in the search order to
ensure that the zero mv is always checked in the RTC coding mode.
It improves the average speed -6 compression performance by 0.3%
in both PSNR and SSIM at no visible speed change.

Change-Id: I465a7e59f4e20cd84fee3f02ced6f98036945949