Support decoding with external JPEG tables

[targos]

Hi! I'm looking into supporting the Photoshop JPEG extension in my TIFF decoder and would love to be able to use jpeg-js under the hood!
However, I am currently blocked because the TIFF JPEG compression allows to specify JPEG quantization and/or Huffman tables in a field separated from the main JPEG data. Would you be open to adding support for that in jpeg-js? I can work on it, but would like to be sure it will be accepted before I do, and maybe get some help on how to expose it in the API.

[patrickhulce]

Hey @targos neat project thanks for filing :)

We're pretty risk adverse at this stage in the project's life, so if you think it can be pulled off with minimal refactoring of the existing decoding path and a completely separate set of methods in the API for invoking the individual pieces you need, then I'm happy to land, but if it starts to get too murky and/or lots of state or flow changes we probably wouldn't accept it.

I think it should be doable to expose a bit of the underlying logic out into separate methods we can export. You might need to piece together the flow things yourself in tiff though.