convert method
- Buffer<
void> input 
override
    Converts input and returns the result of the conversion.
Implementation
@override
Uint8List convert(Buffer<void> input) {
  // Verify the resolution
  RangeError.checkValueInInterval(
    input.width,
    1,
    _maxResolution,
    'input.width',
  );
  RangeError.checkValueInInterval(
    input.height,
    1,
    _maxResolution,
    'input.height',
  );
  return _encodeUncompressedPng(input.mapConvert(_pngNativeFormat));
}