[libpng16] Implemented 'restrict' for png_info and png_struct.

Added new "png_structrp" typedef. Because of the
way libpng works both png_info and png_struct are always accessed via a
single pointer.  This means adding C99 'restrict' to the pointer gives
the compiler some opportunity to optimize the code.  This change allows that.

example.c

@@ -39,6 +39,7 @@
 #include <string.h>
 #include <stdio.h>
 #include <png.h>
+#include <zlib.h>
 
 int main(int argc, const char **argv)
 {