Blender Git Loki

Fix (unreported) broken BLI_task's forloop func in case we have less iterations that workers.

When called with very small range, `BLI_task_parallel_range_ex()` would generate a zero `chunk_size`,
leading to some infinite looping in `parallel_range_func` due to `parallel_range_next_iter_get` returning
true without actually increasing the counter!

So now, we ensure `chunk_size` and `num_tasks` are always at least 1 (and avoid generating too much tasks too).