RFR: JDK-8295685: Update Libpng to 1.6.38

[Harshitha Onkar]

Updated libpng library to v1.6.38.

Following files have been updated -

- 12 source code files of libpng 
-  LICENSE, README, CHANGES files
- legal/libpng.md

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

Commit messages:
 - changed to unix style line endings
 - libpng update - v1.6.38

Changes: https://git.openjdk.org/jdk/pull/10953/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10953&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295685
  Stats: 205 lines in 16 files changed: 83 ins; 33 del; 89 mod
  Patch: https://git.openjdk.org/jdk/pull/10953.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10953/head:pull/10953

PR: https://git.openjdk.org/jdk/pull/10953