message: gltfpack: Fix PNG dimension offsets
  This is a fallout from
diff: @@ -137,8 +137,8 @@ static bool getDimensionsPng(const std::string& data, int& width, int& height)
  if (data.compare(12, 4, "IHDR") != 0)
  return false;
  - width = readInt32(data, 18);
  - height = readInt32(data, 22);
  + width = readInt32(data, 16);
  + height = readInt32(data, 20);
  return true;
  }
  