[jdk8u-dev] RFR: 8295685: Update Libpng to 1.6.38

[Martin Balao]

Hi all,

This pull request contains a backport of commit [d183dc25f7360c3012726acf8c03874df6fc41a4](https://github.com/openjdk/jdk11u-dev/commit/d183dc25f7360c3012726acf8c03874df6fc41a4) from the [openjdk/jdk11u-dev](https://github.com/openjdk/jdk11u-dev) repository.

This backport is a follow up fix of [JDK-8209115](https://bugs.openjdk.org/browse/JDK-8209115), and a dependency of [JDK-8305815](https://bugs.openjdk.org/browse/JDK-8305815).

This backport applied cleanly, except for the changes to the THIRD_PARTY_README files that are specific to 8u.

Note: Changes to src/java.desktop/share/native/libsplashscreen/libpng/UPDATING.txt were skipped because this file was not introduced by the 8u backport of 8208353. That's okay, as for 8u we will backport libpng from 11u or newer JDK releases, and not directly from the library.

Thanks!

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/371

Commit messages:
 - Backport d183dc25f7360c3012726acf8c03874df6fc41a4

Changes: https://git.openjdk.org/jdk8u-dev/pull/372/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295685
  Stats: 624 lines in 22 files changed: 444 ins; 33 del; 147 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/372.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/372/head:pull/372

PR: https://git.openjdk.org/jdk8u-dev/pull/372