Check libpng directly into third_party/

third_party/libpng/libpngpf.3

Index: third_party/libpng/libpngpf.3
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+.TH LIBPNGPF 3 "May 14, 2016"
+.SH NAME
+libpng \- Portable Network Graphics (PNG) Reference Library 1.6.22rc01
+(private functions)
+.SH SYNOPSIS
+\fB#include \fI"pngpriv.h"
+
+\fBAs of libpng version \fP\fI1.5.1\fP\fB, this section is no longer \fP\fImaintained\fP\fB, now that the private function prototypes are hidden in pngpriv.h and not accessible to applications. Look in pngpriv.h for the prototypes and a short description of each \fIfunction.
+
+.SH DESCRIPTION
+The functions previously listed here are used privately by libpng and are not
+available for use by applications.  They are not "exported" to applications
+using shared libraries.
+
+.SH SEE ALSO
+.BR "png"(5), " libpng"(3), " zlib"(3), " deflate"(5), " " and " zlib"(5)
+.SH AUTHOR
+Glenn Randers-Pehrson