Introduced by 9c337fcfe2 mistaken that `MeshExtract.use_threading` set
to false means that no threading is used at all. This is not the case it
will still perform threading for large objects, it will only run the
different subtasks in serial.
Fixed by scheduling the `lines_loose` in the task_pool after the rest
have been executed. This is cleaner than the previous implementation as
it sticks more to the actual design.
f4b7d57551