FPII-2034: DO NOT MERGE Update libpng to 1.6.20

DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I2fc2106965c789a54f04412e9c63a0901b0c1e71
(cherry picked from commit 0b39f202aaedd3029e4270d2af7d58ba6634f9cd)
diff --git a/tests/pngvalid-gamma-16-to-8 b/tests/pngvalid-gamma-16-to-8
index b322338..074945d 100755
--- a/tests/pngvalid-gamma-16-to-8
+++ b/tests/pngvalid-gamma-16-to-8
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngvalid --gamma-16-to-8
+exec ./pngvalid --strict --gamma-16-to-8