DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngimage-full b/tests/pngimage-full
index f5eb989..1198ed0 100755
--- a/tests/pngimage-full
+++ b/tests/pngimage-full
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngimage --exhaustive --log "${srcdir}/contrib/pngsuite/"*.png
+exec ./pngimage --exhaustive --list-combos --log "${srcdir}/contrib/pngsuite/"*.png