DO NOT MERGE Update libpng to 1.6.20

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

image_transform_test_counter(png_uint_32 counter, unsigned int max)
{
 /* Test the list to see if there is any point contining, given a current
    * counter and a 'max' value.
    */
   image_transform *next = image_transform_first;

 while (next != &image_transform_end)
 {
 /* For max 0 or 1 continue until the counter overflows: */
      counter >>= 1;

 /* Continue if any entry hasn't reacked the max. */
 if (max > 1 && next->local_use < max)
 return 1;
      next = next->list;
 }

 return max <= 1 && counter == 0;

 }

image_transform_test_counter(png_uint_32 counter, unsigned int max)
{
 /* Test the list to see if there is any point contining, given a current
    * counter and a 'max' value.
    */
   image_transform *next = image_transform_first;

 while (next != &image_transform_end)
 {
 /* For max 0 or 1 continue until the counter overflows: */
      counter >>= 1;

 /* Continue if any entry hasn't reacked the max. */
 if (max > 1 && next->local_use < max)
 return 1;
      next = next->list;
 }

 return max <= 1 && counter == 0;

 }
