DO NOT MERGE Update libpng to 1.6.20

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

sbit_modification_init(sbit_modification *me, png_modifier *pm, png_byte sbit)
{
   modification_init(&me->this);
   me->this.chunk = CHUNK_sBIT;
   me->this.modify_fn = sbit_modify;
   me->this.add = CHUNK_PLTE;
   me->sbit = sbit;
   me->this.next = pm->modifications;
   pm->modifications = &me->this;
}

sbit_modification_init(sbit_modification *me, png_modifier *pm, png_byte sbit)
{
   modification_init(&me->this);
   me->this.chunk = CHUNK_sBIT;
   me->this.modify_fn = sbit_modify;
   me->this.add = CHUNK_PLTE;
   me->sbit = sbit;
   me->this.next = pm->modifications;
   pm->modifications = &me->this;
}
