RFR: JDK-8329004 : Update Libpng to 1.6.43

[Harshitha Onkar]

Libpng updated from 1.6.40 to  1.6.43

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Commit messages:
 - .md file update
 - libpng update v1.6.43

Changes: https://git.openjdk.org/jdk/pull/18964/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18964&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329004
  Stats: 694 lines in 23 files changed: 258 ins; 205 del; 231 mod
  Patch: https://git.openjdk.org/jdk/pull/18964.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18964/head:pull/18964

PR: https://git.openjdk.org/jdk/pull/18964