[jdk8u-dev] RFR: 8295685: Update Libpng to 1.6.38 [v3]

[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!

Martin Balao has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'JDK-8295685' of https://github.com/martinuy/jdk8u-dev into JDK-8295685
 - Removal of the markdown syntax as it's not in the original libpng AUTHORS file.
 - Backport d183dc25f7360c3012726acf8c03874df6fc41a4
 - Backport e4fdd0391733756f5b898371a66b38869d625c77
 - Removal of the markdown syntax as it's not in the original libpng AUTHORS file.
 - Backport d183dc25f7360c3012726acf8c03874df6fc41a4

-------------

Changes:
  - all: https://git.openjdk.org/jdk8u-dev/pull/372/files
  - new: https://git.openjdk.org/jdk8u-dev/pull/372/files/3fa3cfaf..76485d11

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=372&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=372&range=01-02

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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