Based on usages so far: - Split callback worker func in two, 'basic' and 'extended' versions. The former goes back to the simplest verion, while the later keeps the 'userdata_chunk', and gets the thread_id too. - Add use_threading to simple BLI_task_parallel_range(), turns out we need this pretty much systematically, and allows to get rid of most usages of BLI_task_parallel_range_ex(). - Now BLI_task_parallel_range() expects 'basic' version of callback, while BLI_task_parallel_range_ex() expectes 'extended' version of the callback. All in all, this should make common usage of BLI_task_parallel_range simpler (less verbose), and add access to advanced callback to thread id, which is mandatory in some (future) cases.