message: gltfpack: Update DFD for images with alpha
  DFD for ETC1S images seems to be in continuous flux :( This change
  updates it again to match the current validation code in ktx2check.
diff: @@ -139,7 +139,13 @@ static void createDfd(std::vector<uint32_t>& result, bool alpha, bool srgb, bool
  KHR_DFDSETSVAL(dfd, 0, CHANNELID, KHR_DF_CHANNEL_ETC1S_RGB);
  if (alpha)
  + {
  KHR_DFDSETSVAL(dfd, 1, CHANNELID, KHR_DF_CHANNEL_ETC1S_AAA);
  +
  + // Every decoded alpha block is 8 bytes = 64 bits (encoded as 63)
  + KHR_DFDSETSVAL(dfd, 1, BITLENGTH, 63);
  + KHR_DFDSETSVAL(dfd, 1, BITOFFSET, 64);
  + }
  }
  for (int i = 0; i < sample_count; ++i)
  