RFR: JDK-8348598 : Update Libpng to 1.6.47

[Harshitha Onkar]

Upgraded Libpng to v1.6.47

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Commit messages:
 - unused-function added for gcc
 - libpng upgrade

Changes: https://git.openjdk.org/jdk/pull/24021/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24021&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348598
  Stats: 4982 lines in 23 files changed: 1913 ins; 2285 del; 784 mod
  Patch: https://git.openjdk.org/jdk/pull/24021.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24021/head:pull/24021

PR: https://git.openjdk.org/jdk/pull/24021