hg: jdk8u/jdk8u-dev/jaxp: 8217676: Upgrade libpng to 1.6.37

[zgu at redhat.com]

Changeset: 187f73676840
Author:    zgu
Date:      2019-09-12 15:21 -0400
URL:       https://hg.openjdk.java.net/jdk8u/jdk8u-dev/jaxp/rev/187f73676840

8217676: Upgrade libpng to 1.6.37
Reviewed-by: prr, jdv, kcr

! THIRD_PARTY_README