[jdk11u-dev] RFR: 8305815: Update Libpng to 1.6.39

[Goetz Lindenmaier]

I backport this for parity with 11.0.21-oracle.

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Commit messages:
 - Backport 14874779eeb9a279479c5aaddb40c056780d6152

Changes: https://git.openjdk.org/jdk11u-dev/pull/1952/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1952&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/jdk11u-dev/pull/1952.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/1952/head:pull/1952

PR: https://git.openjdk.org/jdk11u-dev/pull/1952