[jdk8u] RFR: 8316030: Update Libpng to 1.6.40

[Andrew John Hughes]

Update to the latest version of libpng. Change includes both the backport of the latest libpng change, [JDK-8372534](https://bugs.openjdk.org/browse/JDK-8372534), and previous updates not present in 8u ([JDK-8316030](https://bugs.openjdk.org/browse/JDK-8316030) - 1.6.40, [JDK-8329004](https://bugs.openjdk.org/browse/JDK-8329004) - 1.6.43, [JDK-8348598](https://bugs.openjdk.org/browse/JDK-8348598) - 1.6.47). The change built fine with GCC 8 on RHEL 8 and `java/awt/SplashScreen` tests passed.

I also included `UPDATING.txt` introduced in [JDK-8208353](https://bugs.openjdk.org/browse/JDK-8208353) in 11u, but not 8u, and updated in [JDK-8295685](https://bugs.openjdk.org/browse/JDK-8295685) as part of the JDK-8329004 backport which updates it.

I'm proposing this during rampdown so the release ships with the latest version, as we also do with e.g. tzdata.

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Commit messages:
 - Backport be7966c633830516c703f14610e4fef1bbe7f57d
 - Backport 61227856ccd28d887d56d0c93eabd07cf6b026d3
 - Backport 69b6e6d2ad5f0e07b5718dd8a45a725fe328e67f
 - Backport 8191f621993f5c271e31d18aa385333e39d6a919

Changes: https://git.openjdk.org/jdk8u/pull/83/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u&pr=83&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316030
  Stats: 6267 lines in 23 files changed: 2572 ins; 2571 del; 1124 mod
  Patch: https://git.openjdk.org/jdk8u/pull/83.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u.git pull/83/head:pull/83

PR: https://git.openjdk.org/jdk8u/pull/83