Log Message:
png: update to 1.6.43.

We now have support for eXIf chunks in the "push" reader, thanks to Chris
Blume. On the topic of EXIF support: the "pngexifinfo" experiment, which
had evolved together with the draft specification until it reached its
final form known as "eXIf 2017-05-31", is now available at its new home in
the new subdirectory contrib/pngexif.

Interestingly, we have a fix that is applicable to the 16-bit platform
builds of libpng (which, by the way, are not only still existing but also
still functioning), thanks to John Bowler.

And we have all kinds of fixes and improvements in the build scripts, in
the test suite, and in the CI verification process. Notable among those
improvements is the linting script (ci/ci_lint.sh), running automatically
as a GitHub Action for all contributors. (Thanks, GitHub!)