[jdk21u] Integrated: 8372534: Update Libpng to 1.6.51

[Andrew John Hughes]

On Wed, 17 Dec 2025 15:11:48 GMT, Andrew John Hughes <andrew at openjdk.org> wrote:

> Update to the latest version of libpng. Changes from `ClientLibraries.gmk` had to be moved to `Awt2dLibraries.gmk` but applied as is, despite different file and slightly different context. The change built fine with GCC 8 on RHEL 8.
> 
> I'm proposing this during rampdown so the release ships with the latest version, as we also do with e.g. tzdata.

This pull request has now been integrated.

Changeset: 5c2ab8e0
Author:    Andrew John Hughes <andrew at openjdk.org>
URL:       https://git.openjdk.org/jdk21u/commit/5c2ab8e057ccc77019047f9e7e5c4928f876b555
Stats:     638 lines in 21 files changed: 297 ins; 207 del; 134 mod

8372534: Update Libpng to 1.6.51

Reviewed-by: mbaesken, sgehwolf
Backport-of: cb7fb728d4805ed4e9d62ce31cee7c948f3d5746

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PR: https://git.openjdk.org/jdk21u/pull/478