Concurrent GPU / multithread tasks.

[dlyr]

          My bad, I mixed up between runTask and runTasksInThisThread.

I'll fix it.
The local taskQueue is the most direct solution I find to prevent thread taking tasks during "this thread" run.

When I think about it, the solution might be to have two tasks lists, one for multi-thread, the other for this thread, and they might have inter dependencies.
Hence GPU tasks in main thread and other tasks could be scheduled by the same taskQueue. Let's postpone for a future PR

Originally posted by @dlyr in #1056 (comment)