RFR: JDK-8305815: Update Libpng to 1.6.39

[Harshitha Onkar]

Updated libpng to v1.6.39

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Commit messages:
 - added missing GPLv2 header
 - .md file commit
 - libpng src changes

Changes: https://git.openjdk.org/jdk/pull/13755/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13755&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305815
  Stats: 101 lines in 10 files changed: 15 ins; 1 del; 85 mod
  Patch: https://git.openjdk.org/jdk/pull/13755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13755/head:pull/13755

PR: https://git.openjdk.org/jdk/pull/13755