[jdk21u] RFR: 8372534: Update Libpng to 1.6.51

Wed Dec 17 15:19:00 UTC 2025

Update to the latest version of libpng. Changes from `ClientLibraries.gmk` had to be moved to `Awt2dLibraries.gmk` but applied as is, despite different file and slightly different context. 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 cb7fb728d4805ed4e9d62ce31cee7c948f3d5746

Changes: https://git.openjdk.org/jdk21u/pull/478/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=478&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/jdk21u/pull/478.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/478/head:pull/478

PR: https://git.openjdk.org/jdk21u/pull/478