task_set : add wait_for_task (#1847)
task_set : add wait_for_task (#1847)

This lets you wait for just one task in the task set to finish. Versus
the usual wait() that waits for all tasks in the set to be completed.

This is handy for "pipelining", where you are using the tasks as a
queue, and as they are done, you can pass them off to the next
stage. There's no reason to wait for all to complete to begin processing
the first-completed tasks coming out of the queue.
lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio