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