RFR: 8183960: Upgrade to libpng 1.6.34

[Philip Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8183960
webrev: http://cr.openjdk.java.net/~prr/8183960/
This upgrades the libpng used by splashscreen from 1.6.28 to 1.6.34

Built on all supported JDK10 platforms and the (few) SplashScreen reg 
tests that we have pass.

-phil.