Add LodePNG files used by cabi to read PNG files

Added lodepng.c (renamed from lodepng.cpp), lodepng.h and LICENSE from
the LodePNG project by Lode Vandevenne.
https://github.com/lvandeve/lodepng
Since this code is licensed separately from the cabi code, it has been
placed in its own subdirectory named lodepng.

cabi.c and README.md have been modified for the LodePNG file location.

extras/cabi/README.md

@@ -10,7 +10,7 @@ https://github.com/TEAMarg/drawCompressed
 
 to build:
 ```Bash
-gcc cabi.c lodepng.c -o cabi
+gcc cabi.c lodepng/lodepng.c -o cabi
 ```
 
 to use:

extras/cabi/cabi.c

@@ -25,7 +25,7 @@ cabi in.png [array_name_prefix]
 #include <stdlib.h>
 #include <stdio.h>
 #include <memory.h>
-#include "lodepng.h"
+#include "lodepng/lodepng.h"
 
 // alternative pixel order mapping
 //#define READING_ORDER 1

extras/cabi/lodepng/LICENSE

@@ -0,0 +1,21 @@
+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.
+