DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngstest-linear b/tests/pngstest-linear
new file mode 100755
index 0000000..4a5b3fe
--- /dev/null
+++ b/tests/pngstest-linear
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec "${srcdir}/tests/pngstest" linear none