DO NOT MERGE Update libpng to 1.6.20

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

static int rpng2_x_msb(ulg u32val)
{
 int i;

 for (i = 31;  i >= 0; --i) {
 if (u32val & 0x80000000L)
 break;
        u32val <<= 1;
 }
 return i;
}

static int rpng2_x_msb(ulg u32val)
{
 int i;

 for (i = 31;  i >= 0; --i) {
 if (u32val & 0x80000000L)
 break;
        u32val <<= 1;
 }
 return i;
}
