[jdk11u-dev] RFR: 8316030: Update Libpng to 1.6.40

[Andrew Lu]

I backport this for parity with 11.0.23-oracle.

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Commit messages:
 - Backport 158293d2517695f8c5eaca1b46ecf0f1f9f09691

Changes: https://git.openjdk.org/jdk11u-dev/pull/2336/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2336&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316030
  Stats: 246 lines in 11 files changed: 89 ins; 82 del; 75 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2336.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2336/head:pull/2336

PR: https://git.openjdk.org/jdk11u-dev/pull/2336