DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngvalid-gamma-threshold b/tests/pngvalid-gamma-threshold
index 2cfca91..3fe4edf 100755
--- a/tests/pngvalid-gamma-threshold
+++ b/tests/pngvalid-gamma-threshold
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngvalid --gamma-threshold
+exec ./pngvalid --strict --gamma-threshold