Robuta

https://www.codereview.appspot.com/5758043/ Issue 5758043: Blend alpha correctly for ARGB32, ARGB16 (4444), and A8 modes. - Code Review https://codereview.appspot.com/5758043/ Issue 5758043: Blend alpha correctly for ARGB32, ARGB16 (4444), and A8 modes. - Code Review