libvpx

Use average mb energy from first pass in AQ2 test.

AQ2 modified to use mb_av_energy in defining variance
thresholds used alongside complexity when defining the
segment to be used for an SB64.

Slight improvements in metrics (ssim and PSNR).

Change-Id: Idb9cb73f7d9c4f7118cd7e84ac77b0f25cacbf81