Merge the 3 threading models into a single one
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
https://drive.google.com/drive/folders/1dcpjCOL3KZg1xLA4l1Dfi4mCr9vmPlgQ?usp=sharing
Edited by Victorien Le Couviour--Tuffet