RFR: 8375063: Update Libpng to 1.6.54

[Jayathirth D V]

Upgrade libpng used for Splashscreen from 1.6.51 to 1.6.54 version.
Testing is green with this update.

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Commit messages:
 - 8375063: Update Libpng to 1.6.54

Changes: https://git.openjdk.org/jdk/pull/29292/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29292&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375063
  Stats: 1406 lines in 16 files changed: 443 ins; 7 del; 956 mod
  Patch: https://git.openjdk.org/jdk/pull/29292.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29292/head:pull/29292

PR: https://git.openjdk.org/jdk/pull/29292