matched 170 decoded textures to a T8aD flag word
  bit SET   n= 61  mean %(RGB>A)  55.52  median  52.52
  bit clear n=109  mean %(RGB>A)  33.66  median  30.17

  premultiplied would require ~0% for the flagged group -> REFUTED
  best single-threshold accuracy: 76.5% at %(RGB>A) > 83.5
  (base rate, always-guess-majority: 64.1%)