Migrate LodePNG to submodule

We are no longer copy-pasting LodePNG source files directly.

As we can't rename lodepng.cpp to lodepng.c in the submodule itself, we
need to make a wrapper file, lodepng_wrapper.c, that #includes
lodepng.cpp, but gets compiled as C.

third_party/lodepng/LICENSE.txt

@@ -1,21 +0,0 @@
-Copyright (c) 2005-2018 Lode Vandevenne
-
-This software is provided 'as-is', without any express or implied
-warranty. In no event will the authors be held liable for any damages
-arising from the use of this software.
-
-Permission is granted to anyone to use this software for any purpose,
-including commercial applications, and to alter it and redistribute it
-freely, subject to the following restrictions:
-
-    1. The origin of this software must not be misrepresented; you must not
-    claim that you wrote the original software. If you use this software
-    in a product, an acknowledgment in the product documentation would be
-    appreciated but is not required.
-
-    2. Altered source versions must be plainly marked as such, and must not be
-    misrepresented as being the original software.
-
-    3. This notice may not be removed or altered from any source
-    distribution.
-