Update libpng to version 1.6.3

Change-Id: I86cb46ec376eb41324b82d2875757aeda734c5c1
diff --git a/contrib/README.txt b/contrib/README.txt
new file mode 100644
index 0000000..bcd433d
--- /dev/null
+++ b/contrib/README.txt
@@ -0,0 +1,4 @@
+
+This "contrib" directory contains contributions which are not necessarily under
+the libpng license, although all are open source.  They are not part of
+libpng proper and are not used for building the library.