Add Clone to Decoder

[tristan-morris]

Added clone to image-tiff, allowing the Decoder to be cloned and re-used.

Useful for Tiffs located on remote storage, without needing decoding.

[fintelia]

Could you say a bit more about the motivation for this PR?

[tristan-morris]

My motivation was being able to performantly perform concurrent reads into a geotiff. This is most problematic/slow during the initial IFD and header parse as there are many seek and reads which need to be performed. This slowness is most pronounced when a geotiff is hosted on a remote s3 bucket, and opening multiple readers would be very slow.

I'll end up closing this PR as I've since discovered std::fs::File (rust-lang/rfcs#2259) essentially shares the same file handle and so the seek+read breaks down very quickly as the position is moved. Not sure I agree with this design decision behind a cloned Reader sharing the OS file handle, but it makes sense. So simply adding Clone isn't so simple for the 99% use-case :)

[fintelia]

Hm, that does seem like too much of a gotcha to go forward with this PR. I would like to have a better API that was more conducive to parallel decoding, but API design is hard and I haven't been able to devote much time to refactoring/improvements to this crate