move jpeg encode / decode outside libtiff

[jcupitt]

At the moment, libvips does JPEG encode and decode using the standard libtiff mechanisms. This is easy, but it means that JPEG encoding runs inside libtiff, and that means it can't be threaded.

We could handle the encode / decode ourselves and process many tiles in parallel. This could give a nice speedup in many cases, especially for OME tiff.

openslide has sample code for this we could check.

See: https://forum.image.sc/t/how-to-speed-up-the-process-of-ome-tiff-generation-using-multi-processing/69104/3