Lots of dumb crap to do... something. Instead of adding yet another dumb
helper, just use the main" sws_utils API in both callers. (Which,
unfortunately, has been duplicated for glorious webp screenshots,
despite the fact that webp is crap.)
Good part: can enable zimg for screenshots (as far as needed).
Bad part: uses "default" swscale parameters instead of HQ now.