[jdk11u-dev] RFR: 8348598: Update Libpng to 1.6.47

[Antonio Vieiro]

Backport of [JDK-8348598](https://bugs.openjdk.org/browse/JDK-8348598) from JDK-17. 

Not clean as `make/lib/Awt2dLibraries.gmk` is different from JDK-17. Resolved adding the new `unused-function`  to `gcc`and `gclang` disabled warnings for AWT libraries.

Tier 1 tests pass. Tests for `javax.imageio` and `java2d/cmm` pass. `jfc_demo` is great to see! :-) .


==============================
Test summary
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   TEST                                              TOTAL  PASS  FAIL ERROR   
   jtreg:test/jdk/javax/imageio                        197   197     0     0   
==============================
TEST SUCCESS


==============================
Test summary
==============================
   TEST                                              TOTAL  PASS  FAIL ERROR   
   jtreg:test/jdk/sun/java2d/cmm                        19    19     0     0   
==============================
TEST SUCCESS

==============================
Test summary
==============================
   TEST                                              TOTAL  PASS  FAIL ERROR   
   jtreg:test/jdk:jfc_demo                               1     1     0     0   
==============================
TEST SUCCESS

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

Commit messages:
 - Backport 025da55defb4428a0cbc75b24842513011f1220d

Changes: https://git.openjdk.org/jdk11u-dev/pull/3021/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3021&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348598
  Stats: 4891 lines in 20 files changed: 1906 ins; 2195 del; 790 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3021.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3021/head:pull/3021

PR: https://git.openjdk.org/jdk11u-dev/pull/3021