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/pngstest-sRGB b/tests/pngstest-sRGB
new file mode 100755
index 0000000..d846081
--- /dev/null
+++ b/tests/pngstest-sRGB
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec "${srcdir}/tests/pngstest" sRGB none