Don't panic when parsing metadata when custom compression is used

[AlanRace]

Some microscopy file types, such as .svs (https://openslide.org/formats/aperio/) are essentially .tif files, with custom tags. I wanted to read in the metadata (width, height etc.), but this failed due to the file using a custom compression scheme (JPEG2000) that is not supported in the image-tiff library/.tif specification.

This PR simply adds in the ability to read in custom compression schemes and therefore also the metadata, without an error occuring.

[fintelia]

What happens if you try to read the file contents when an unsupported scheme is used? Do we still properly return an error?

[AlanRace]

Good question - yes. When running the following code:

let svs_file = File::open(svs_path)?;
let mut decoder = Decoder::new(BufReader::new(svs_file))?;

println!("Dimensions: {:?}", decoder.dimensions());

let chunk = decoder.read_chunk(0)?;

I get:

Dimensions: Ok((39840, 28523))
Error: UnsupportedError(UnsupportedCompressionMethod(Unknown(33005)))