[jdk8u] RFR: 8372534: Update Libpng to 1.6.51

[Alexey Bakhtin]

On Sat, 27 Dec 2025 18:26:18 GMT, Andrew John Hughes <andrew at openjdk.org> wrote:

> Update to the latest version of libpng. Change includes both the backport of the latest libpng change, [JDK-8372534](https://bugs.openjdk.org/browse/JDK-8372534), and previous updates not present in 8u ([JDK-8316030](https://bugs.openjdk.org/browse/JDK-8316030) - 1.6.40, [JDK-8329004](https://bugs.openjdk.org/browse/JDK-8329004) - 1.6.43, [JDK-8348598](https://bugs.openjdk.org/browse/JDK-8348598) - 1.6.47). The change built fine with GCC 8 on RHEL 8 and `java/awt/SplashScreen` tests passed.
> 
> I also included `UPDATING.txt` introduced in [JDK-8208353](https://bugs.openjdk.org/browse/JDK-8208353) in 11u, but not 8u, and updated in [JDK-8295685](https://bugs.openjdk.org/browse/JDK-8295685) as part of the JDK-8329004 backport which updates it.
> 
> I'm proposing this during rampdown so the release ships with the latest version, as we also do with e.g. tzdata.

Marked as reviewed by abakhtin (Committer).

Thank you for the backport.
I'm not a reviewer, but it looks good to me. I do not see any regressions.

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PR Review: https://git.openjdk.org/jdk8u/pull/83#pullrequestreview-3629185572
PR Comment: https://git.openjdk.org/jdk8u/pull/83#issuecomment-3712934159