JXL; multithread for encoding

[nji9]

In XnViewMP's actual version it encodes JXL single-threaded;
for lossy encoding this takes about twice as much time as if multi-thread enabled
(no difference for lossless).

EDIT:
lossy/ lossless

[Kadet]

+1
We need multi-thread for coding JXL.

[nji9]

Checked in v. 1.4.3
... still doesn't support it.
"MP" stands for "Multi Processor"?

When used with standard option -q 90
it takes half as long to encode
when multithreaded.
The lib supports it - by default.

[revadiaz]

+1 hopefully this will be implemented soon.