[jdk17u] RFR: 8372534: Update Libpng to 1.6.51

[Matthias Baesken]

On Thu, 18 Dec 2025 14:25:00 GMT, Andrew John Hughes <andrew at openjdk.org> wrote:

> Update to the latest version of libpng. Changes from `Awt2dLibraries.gmk` were dropped as warnings are not split by file in 17u (missing [JDK-8294466](https://bugs.openjdk.org/browse/JDK-8294466)) and there are still unused warnings from `splashscreen_impl.c` in both 17u & 21u. 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.

Maybe you could get rid of the unused-function  warning disabling for gcc in 17 too (see Awt2dLibraries.gmk ) , but up to you!

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Marked as reviewed by mbaesken (Reviewer).

PR Review: https://git.openjdk.org/jdk17u/pull/415#pullrequestreview-3603035730