[PNG] ISO 21496-1 (gain maps) published

Hello everyone,

It looks like ISO 21496-1 (gain maps) has been published
<https://www.iso.org/standard/86775.html>. Congrats to everyone who worked
on that!

It is not yet free, though. Which makes it difficult for us to reference it
in PNG.
There is some work being done to establish a liaison between ISO and W3C so
we can make a formal request for it to be freely available.