newDecoder
fun newDecoder(result: SourceFetchResult, options: Options, imageLoader: ImageLoader, startIndex: Int = 0): Pair<Decoder, Int>?
Create a new Decoder using the registered decoderFactories.
Return
A Pair where the first element is the new Decoder and the second element is the index of the factory in decoderFactories that created it. Returns 'null' if a Decoder cannot be created for result.