Update libpng to version 1.6.3

Change-Id: I86cb46ec376eb41324b82d2875757aeda734c5c1
diff --git a/tests/pngvalid-gamma-threshold b/tests/pngvalid-gamma-threshold
new file mode 100755
index 0000000..2cfca91
--- /dev/null
+++ b/tests/pngvalid-gamma-threshold
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec ./pngvalid --gamma-threshold