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/libpngpf.3 b/libpngpf.3
index 844a9fc..4da0843 100644
--- a/libpngpf.3
+++ b/libpngpf.3
@@ -1,6 +1,6 @@
-.TH LIBPNGPF 3 "March 6, 2014"
+.TH LIBPNGPF 3 "December 3, 2015"
 .SH NAME
-libpng \- Portable Network Graphics (PNG) Reference Library 1.6.10
+libpng \- Portable Network Graphics (PNG) Reference Library 1.6.20
 (private functions)
 .SH SYNOPSIS
 \fB#include \fI"pngpriv.h"