[jdk11u-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/jdk11u-dev/pull/2743/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2743&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/jdk11u-dev/pull/2743.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2743/head:pull/2743

PR: https://git.openjdk.org/jdk11u-dev/pull/2743