png: update to disable processor optimization

ThirdParty/png/update.sh

@@ -8,7 +8,7 @@ readonly name="png"
 readonly ownership="Libpng Upstream <kwrobot@kitware.com>"
 readonly subtree="ThirdParty/$name/vtk$name"
 readonly repo="https://gitlab.kitware.com/third-party/png.git"
-readonly tag="for/vtk-20181015-1.6.35"
+readonly tag="for/vtk-20190528-1.6.35"
 readonly paths="
 .gitattributes
 CMakeLists.vtk.txt