[jdk21u] RFR: 8372534: Update Libpng to 1.6.51

[Matthias Baesken]

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.

Marked as reviewed by mbaesken (Reviewer).

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PR Review: https://git.openjdk.org/jdk21u/pull/478#pullrequestreview-3591479129