[jdk25u-dev] RFR: 8372534: Update Libpng to 1.6.51

Wed Dec 3 14:18:48 UTC 2025

Update to the latest version of libpng. Clean backport which built fine with GCC 11 on RHEL 8.

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Commit messages:
 - Backport c7a489db9e4a7d696623fc2155a5504d9d2adb0d

Changes: https://git.openjdk.org/jdk25u-dev/pull/56/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=56&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372534
  Stats: 638 lines in 21 files changed: 297 ins; 207 del; 134 mod
  Patch: https://git.openjdk.org/jdk25u-dev/pull/56.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/56/head:pull/56

PR: https://git.openjdk.org/jdk25u-dev/pull/56