[jdk17u] RFR: 8372534: Update Libpng to 1.6.51

Thu Dec 18 14:32:13 UTC 2025

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.

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Commit messages:
 - Backport 5c2ab8e057ccc77019047f9e7e5c4928f876b555

Changes: https://git.openjdk.org/jdk17u/pull/415/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u&pr=415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372534
  Stats: 636 lines in 20 files changed: 297 ins; 206 del; 133 mod
  Patch: https://git.openjdk.org/jdk17u/pull/415.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u.git pull/415/head:pull/415

PR: https://git.openjdk.org/jdk17u/pull/415