Check libpng directly into third_party/

third_party/libpng/contrib/pngminim/encoder/pngusr.h

Index: third_party/libpng/contrib/pngminim/encoder/pngusr.h
diff --git a/third_party/libpng/contrib/pngminim/encoder/pngusr.h b/third_party/libpng/contrib/pngminim/encoder/pngusr.h
new file mode 100644
index 0000000000000000000000000000000000000000..997d44fe9040de6e96cb21bdb6bee45052fbe0f1
--- /dev/null
+++ b/third_party/libpng/contrib/pngminim/encoder/pngusr.h
@@ -0,0 +1,23 @@
+/* minwrpngconf.h: headers to make a minimal png-write-only library
+ *
+ * Copyright (c) 2007, 2010-2013 Glenn Randers-Pehrson
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ *
+ * Derived from pngcrush.h, Copyright 1998-2007, Glenn Randers-Pehrson
+ */
+
+#ifndef MINWRPNGCONF_H
+#define MINWRPNGCONF_H
+
+/* To include pngusr.h set -DPNG_USER_CONFIG in CPPFLAGS */
+
+/* List options to turn off features of the build that do not
+ * affect the API (so are not recorded in pnglibconf.h)
+ */
+
+#define PNG_ALIGN_TYPE PNG_ALIGN_NONE
+
+#endif /* MINWRPNGCONF_H */