Fix to avoid getting stuck at very low Q even
though content is changing, which can happen for --min-q=0.

Fix is to more aggressively increase active_worst_quality
when detecting significant rate_deviation at very low Q.

Change will only affect 1 pass VBR for --min-q < 4, so no
change in ytlive metrics for --min-q >= 4.

Change-Id: I4dd77dd7c08a30a4390da0ff2c8bda6fccfa76d7