[PNG] Chrome adds support for cLLi and mDCv chunks

Hello everyone,

Chrome just landed a change which adds support for the cLLi and mDCv chunks
<https://chromium-review.googlesource.com/c/chromium/src/+/4748191>.
Thank you to Sami Boukortt for writing it.

It is useful for us to gather implementation experience. We want to make
sure the changes we propose are reasonable and straightforward. Chrome
already had infrastructure setup for ST 2086 and CTA 861.3. This meant
Chrome's implementation of these chunks may have been easier than another
product which would have needed to add this infrastructure as well. But as
far as the PNG-specific details, the chunk implementations seemed fairly
simple.