Avoid killing textures unless we have several mb

This allows us to decimate less.  The limit of 16 MB was pulled out of my nose, but doesn't seem unreasonable.  Wipeout Pure, for example, is around 7-9 iirc.

We could potentially make this more dynamic as suggested in #6323.  This change already might help that as it may loop through the list even less often?

-[Unknown]