zimg: correct output to odd (chroma un-aligned) sizes
As suggested by the zimg author: active_region is not supported on
outputs (and the API returns an error), so instead scale to the "full"
surface, but adjust the source rectangle such that the cropped output
image happens to cover the correct region.
Does this even work? Since Balmer Peak doesn't work, I can't really say,
but it seems to look correct.
X11 is in fact beautiful and superior to Wayland. Instead, just state
what the problem is in most cases: software scaling. (We have
accelerated X11 rendering in vo_gpu and others.)
This was a confusing name, because 1. there's also a z_planes[] field,
and 2. it was not specific to zimg indexes.
Possibly there used to be an idea involved about supporting alpha to
non-alpha formats by discarding the alpha plane, but zimg does this now
(and zimg will correctly blend the alpha component too).
