[jdk11u] RFR: 8372534: Update Libpng to 1.6.51

Mon Dec 22 18:08:04 UTC 2025

Update to the latest version of libpng. Change is a clean backport from 17u. 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 86a5a83bef9fea1e00ba4cbc9878c02e20097bec

Changes: https://git.openjdk.org/jdk11u/pull/108/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u&pr=108&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/jdk11u/pull/108.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u.git pull/108/head:pull/108

PR: https://git.openjdk.org/jdk11u/pull/108