Blender Git Loki

Fix ugly mistake in BLI_task - freeing while some tasks are still being processed.

Freeing pool was calling `BLI_task_pool_stop()`, which only clears
pool's tasks that are in TODO queue, whithout ensuring no more tasks
from that pool are being processed in worker threads.

This could lead to use-after-free random (and seldom) crashes.

Now use instead `BLI_task_pool_cancel()`, which does waits for all tasks
being processed to finish, before returning.