[jdk21u-dev] RFR: 8329004: Update Libpng to 1.6.43

[Martin Doerr]

Clean backport of [JDK-8329004](https://bugs.openjdk.org/browse/JDK-8329004).

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Commit messages:
 - Backport 4e4229438ad2e8ac59ac675465e4d3d4e13bf156

Changes: https://git.openjdk.org/jdk21u-dev/pull/636/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=636&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329004
  Stats: 610 lines in 18 files changed: 258 ins; 120 del; 232 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/636.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/636/head:pull/636

PR: https://git.openjdk.org/jdk21u-dev/pull/636