DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngvalid-transform b/tests/pngvalid-transform
new file mode 100755
index 0000000..c7536ae
--- /dev/null
+++ b/tests/pngvalid-transform
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec ./pngvalid --strict --transform