libpng v1.6.36, with libpng license v2, almost ready

Hi,

The libpng tip of tree has been very stable, and I consider it ready
to go public.

I have not incorporated all of the changes submitted or requested
(e.g. on GitHub), but there should be room for those, in v1.6.37 or
later.

This is the first-ever libpng to ship with license v2. See the commit
e79085a172 (currently, the top commit). I discussed the license with
the people at OSI. There are no changes to the license text since I
had originally started the discussion, although I was given some
highly useful information. There may be a formal OSI review, but only
after having the license legally reviewed outside OSI.

Meanwhile, if everything goes well, we should have an official
libpng-1.6.36 release, within a week or so.

Sincerely,
Cosmin