[jdk17u] RFR: 8372534: Update Libpng to 1.6.51

[khanbilal732]

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.

Marked as reviewed by khanbilal732 at github.com (no known OpenJDK username).

Marked as reviewed by khanbilal732 at github.com (no known OpenJDK username).

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PR Review: https://git.openjdk.org/jdk17u/pull/415#pullrequestreview-3601225286
PR Review: https://git.openjdk.org/jdk17u/pull/415#pullrequestreview-3601225436