dep: Add libpng 1.6.43

dep/libpng/AUTHORS

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+PNG REFERENCE LIBRARY AUTHORS
+=============================
+
+This is the list of PNG Reference Library ("libpng") Contributing
+Authors, for copyright and licensing purposes.
+
+ * Adam Richter
+ * Andreas Dilger
+ * Chris Blume
+ * Cosmin Truta
+ * Dave Martindale
+ * Eric S. Raymond
+ * Gilles Vollant
+ * Glenn Randers-Pehrson
+ * Greg Roelofs
+ * Guy Eric Schalnat
+ * James Yu
+ * John Bowler
+ * Kevin Bracey
+ * Magnus Holmgren
+ * Mandar Sahastrabuddhe
+ * Mans Rullgard
+ * Matt Sarett
+ * Mike Klein
+ * Pascal Massimino
+ * Paul Schmidt
+ * Philippe Antoine
+ * Qiang Zhou
+ * Sam Bushell
+ * Samuel Williams
+ * Simon-Pierre Cadieux
+ * Tim Wegner
+ * Tom Lane
+ * Tom Tanner
+ * Vadim Barkov
+ * Willem van Schaik
+ * Zhijie Liang
+ * Apple Inc.
+    - Zixu Wang (王子旭)
+ * Arm Holdings
+    - Richard Townsend
+ * Google Inc.
+    - Dan Field
+    - Leon Scroggins III
+    - Matt Sarett
+    - Mike Klein
+    - Sami Boukortt
+    - Wan-Teh Chang
+ * Loongson Technology Corporation Ltd.
+    - GuXiWei (顾希伟)
+    - JinBo (金波)
+    - ZhangLixia (张利霞)
+
+The build projects, the build scripts, the test scripts, and other
+files in the "projects", "scripts" and "tests" directories, have
+other copyright owners, but are released under the libpng license.
+
+Some files in the "ci" and "contrib" directories, as well as some
+of the tools-generated files that are distributed with libpng, have
+other copyright owners, and are released under other open source
+licenses.