Function default_decoder 

pub fn default_decoder<D>() -> Decoder<D, GrayImage, String>
where
    D: GenericImageView<Pixel = Rgba<u8>>,

Create a default Decoder

It will use the following components:

• prepare: BlockedMean
• detect: LineScan
• extract: QRExtractor
• decode: QRDecoder

This is meant to provide a good balance between speed and accuracy