Add support for PNG third edition (HDR)

As the PNG third edition explainer https://github.com/w3c/png/blob/main/Third_Edition_Explainer.md explains, the third edition of png add mainly two things:

APNG (finally official now 17 years after it was introduced) and HDR support.

As I understand it APNG is adopted unchanged and the wuffs implementation already supports that so you are already half-way to supporting PNG third edition.

The last thing remaining is the HDR support, which would be very nice to have since HDR is booming and becoming more and more used.
Chromium already have support for it.