This commit switches meshdeform modifier to use threads to evaluate
the vertices positions using the central task scheduler.
SO now we've got an utility function to help splitting the for loop
into tasks using BLI_task module which is pretty straightforward to
use: it gets range (which is an integer lower and higher bounds) and
the function and userdata to be invoked for each of the iterations.
The only weak point for now is the passing the data to the callback,
this isn't so trivial to improve in pure C.
Reviewers: campbellbarton
Differential Revision: https://developer.blender.org/D838
Replaces ThreadedWorker and is gonna to be used
for threaded object update in the future and
some more upcoming changes.
But in general, it's to be used for any task
based subsystem in Blender.
Originally written by Brecht, with some fixes
and tweaks by self.
