Revert "Task scheduler: Avoid mutex lock in number manipulation functions"

Appears mutex was guarateeing number of tasks is not modified at moments
when it's not expected. Removing those mutexes resulted in some hard-to-catch
locks where worker thread were waiting for work by all the tasks were already
done.

This reverts commit a1d8fe052c.

source/blender/blenlib/intern/task.c

@@ -230,21 +230,28 @@ static void task_free(TaskPool *pool, Task *task, const int thread_id)
 
 static void task_pool_num_decrease(TaskPool *pool, size_t done)
 {
+	BLI_mutex_lock(&pool->num_mutex);
+
 	BLI_assert(pool->num >= done);
 
-	atomic_sub_z((size_t*)&pool->num, done);
+	pool->num -= done;
 	atomic_sub_z(&pool->currently_running_tasks, done);
-	atomic_add_z((size_t*)&pool->done, done);
+	pool->done += done;
 
-	if (pool->num == 0) {
+	if (pool->num == 0)
 		BLI_condition_notify_all(&pool->num_cond);
-	}
+
+	BLI_mutex_unlock(&pool->num_mutex);
 }
 
 static void task_pool_num_increase(TaskPool *pool)
 {
-	atomic_add_z((size_t*)&pool->num, 1);
+	BLI_mutex_lock(&pool->num_mutex);
+
+	pool->num++;
 	BLI_condition_notify_all(&pool->num_cond);
+
+	BLI_mutex_unlock(&pool->num_mutex);
 }
 
 static bool task_scheduler_thread_wait_pop(TaskScheduler *scheduler, Task **task)