[jdk8u-dev] RFR: 8348598: Update Libpng to 1.6.47

[Zdenek Zambersky]

This backport updates bundled libpng to 1.6.47.
There are some differences to jdk11:
- there is no `libpng.md` in JDK 8, equivalent changes were done to `THIRD_PARTY_README`
- changes to `UPDATING.txt` were excluded (this file is not present in 8)
- changes to `/lib/Awt2dLibraries.gmk` were excluded, as build system of jdk 8 is different and there is no support for `DISABLED_WARNINGS_*` there

Otherwise, when it comes to library source files (libpng dir), changes applied cleanly (although they have different path).
I have also tried to use diff to compare contents of libpng directory (after update to 1.6.47) in jdk 8 and jdk 11, there are same, except for `UPDATING.txt`, which is only present in jdk 11.

**Motivation:**
- to fix one build error on macos-15 (it no longer ships with `fp.h`)

 In file included from /Users/runner/work/jdk8u-dev/jdk8u-dev/jdk/jdk/src/share/native/sun/awt/libpng/png.c:42:
/Users/runner/work/jdk8u-dev/jdk8u-dev/jdk/jdk/src/share/native/sun/awt/libpng/pngpriv.h:552:16: fatal error: 'fp.h' file not found
  552 | #      include <fp.h>
      |                ^~~~~~
1 error generated.
gmake[2]: *** [lib/Awt2dLibraries.gmk:1256: /Users/runner/work/jdk8u-dev/jdk8u-dev/jdk/build/macos-x64/jdk/objs/libsplashscreen/png.o] Error 1

- to prepare for [JDK-8372534](https://bugs.openjdk.org/browse/JDK-8372534) (update to 1.6.51) in the future

Testing:
- testing in GHA: OK

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

Commit messages:
 - 8348598: Update Libpng to 1.6.47
 - 8329004: Update Libpng to 1.6.43
 - 8316030: Update Libpng to 1.6.40

Changes: https://git.openjdk.org/jdk8u-dev/pull/729/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=729&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348598
  Stats: 5620 lines in 20 files changed: 2220 ins; 2368 del; 1032 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/729.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/729/head:pull/729

PR: https://git.openjdk.org/jdk8u-dev/pull/729