DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

PNG_FUNCTION(void, (PNGCBAPI error), (png_structp png_ptr, const char *message),
 static PNG_NORETURN)
{
   display *d = (display*)png_get_error_ptr(png_ptr);

   fprintf(stderr, "%s(%s): libpng error: %s\n", d->file, d->test, message);
   display_exit(d);
}

PNG_FUNCTION(void, (PNGCBAPI error), (png_structp png_ptr, const char *message),
 static PNG_NORETURN)
{
   display *d = (display*)png_get_error_ptr(png_ptr);

   fprintf(stderr, "%s(%s): libpng error: %s\n", d->file, d->test, message);
   display_exit(d);
}
