Fix T46093: Thumbnails/previews of materials/textures not displaying in Blender filebrowser when only one thread is available.

Using the global scheduler here is not a really good idea - `filelist_cache_previewf()` is not a short task
that run once, but it's a loop that keeps cheking for work in a TODO queue. This means it won't quickly allow other tasks
to start, so it should not be in the global scheduler.

In fact, asynchronous tasks (that is, tasks that will live for quite a bit of time, and often sleep a lot) should never use
global scheduler, they would steal computing resources from heavy-duty, short-time living ones - and possibly even completely
stall threaded tasks (if all worker threads are executing long-life tasks...).

We could probably even completely bypass the scheduler/task thing here (and directly use threads), but it does not have
that much of an over-head, and still offers easy handling of threading stuff...

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