Add clause to alt ref group breakout.

Add a clause to the breakout test for alt ref groups that
examines the size of the accumulated second reference
frame error compared to the cost of intra coding.

This clause causes a reduction in the average group length for many
clips. Alongside the change to the group length the minimum
boost is increased.

On balance the results are positive for psnr and psnr-hvs
but is negative for ssim/fast ssim for the smaller image formats.

Strong gains on some harder clips (eg ducks take off (midres) ~20%,
husky (lowres) 6-17%. Most of the negative cases are lower motion
clips. Subsequent patch hopefully will help with those.

Change-Id: Ic1f5dbb9153d5089e58b1540470e799f91a65dc4