Blender Git Loki

BLI_task: add support for full-background taskpools.

With current code, in single-threaded context, a pool of task may never be executed
until one calls BLI_task_pool_work_and_wait() on it, this is not acceptable for
asynchronous tasks where you never want to actually lock the main thread.

This commits adds an extra thread in single-threaded case, and a new 'type' of pool,
such that one can create real background pools of tasks. See code for details.

Review: D1565