Merge the 3 threading models into a single one

Merged Victorien Le Couviour--Tuffet requested to merge psilokos/dav1d:task-threading into master

Merges the 3 threading parameters into a single --threads= argument.

Frame threading can still be controlled with the --framedelay= argument.

Internally, the threading model is now a global thread/task pool design.

Co-authored-by: @rbultje

Edited by Victorien Le Couviour--Tuffet

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