RFR: JDK-8305815: Update Libpng to 1.6.39

[Phil Race]

On Tue, 2 May 2023 17:43:46 GMT, Harshitha Onkar <honkar at openjdk.org> wrote:

> Updated libpng from 1.6.38 to 1.6.39.
> 
> - libpng.md file updated accordingly
> - src code files updated from upstream
> 
> CIient (CI Testing) and SplashScreen related tests work as expected with the changes.

Marked as reviewed by prr (Reviewer).

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

PR Review: https://git.openjdk.org/jdk/pull/13755#pullrequestreview-1411660792