<AWT Dev> RFR: 8167228: Update to libpng 1.6.28

[Phil Race]

Webrev: http://cr.openjdk.java.net/~prr/8167228/

This fix updates libpng in JDK 9 to the current version 1.6.28.
Aside from updating to these files there are few changes to note

(1) The CHANGES file has previously inexplicably included a GPL and 
Oracle Copyright.
Nothing whatsoever in here is from Oracle and it is not even source code.
I deleted that notice.

(2) We have always compiled without "WRITE" support, yet included all 
the source
files that are used only for writing. libpng is used only by 
splashscreen (it is compiled
into the library) and that has no need for write support anyway.
So I decided we should stop importing these files to remove any future 
questions
about issues in PNG writing ...
Note that the options to disable WRITE support are configured via 
pnglibconf.h,
more on that in a moment.

(3) In addition pngtest.c is removed. This is wholly unnecessary()

(4) pnglibconf.h is also updated to no longer automatically decode certain
compressed chunks which we don't use anyway ... this will improve 
performance
and robustness.

Future updates, and indeed any backports to 8u should follow the above.

JPRT has built this for all platforms and all SplashScreen tests have 
passed ..
although I think only a few really test png support.

-phil.