JXL: multithreaded decoding

[Yumi]

I've noticed that decoding large JPEG-XL images is very slow - much slower than what djxl can do. From what I can tell, XnView MP's implementation doesn't enable multi-threading. There's an example here of how to decode - from what I can tell, the main thing is that JxlDecoderSetParallelRunner needs to be called, with an appropriate threading runner, so hopefully not too difficult. (I don't know whether you want to keep the thread pool around, but even just re-creating/destroying it for each image should still usually give better performance)

I see a thread suggesting multithreaded encode - so if that's happening, may as well do it for decode too

Thanks!

[Kadet]

+1

[nji9]

+1

[revadiaz]

+1. Really need this.
Opening jxl file with 6000x4000 resolution take a long time (>2 seconds).

[corr]

+1