Fixes unnatural black flickering in some scenes where encoding noise
pushes the background between 0 and 1 PQ. Adding a 1% PQ cutoff allows
censoring such values to 0.

I could also have hard-coded this constant but I decided it's simple
enough to expose, and it being tunable may help with tracking down
related issues in the future, and also allows users to opt out of this
functionality altogether.

Fixes: https://yabb.jriver.com/interact/index.php/topic,136378.msg950343.html#msg950343