[Enhancement] Use application provided ThreadPool

[MrKepzie]

When an external application already has its own threading technology (be it a thread-pool or w/e), it is hard to wire it to OpenImageIO's own thread-pool.

If we enable multi-threading with the oiio threadpool we get overthreading because a threadpool is a centralized way of managing threads workload.

If we disable multi-threading for OIIO using oiio-threads, we then get no multi-threading for things like parallel_convert_image.

OIIO being a library, it should be able to operate with another application's thread-pool, so that we get the best performance possible out of OIIO.

I tried to hack around and modify the thread-pool API so it could be inherited by the application and we could replace the default thread-pool by forwarding calls to our own threading mechanism, but the API of the thread-pool being complicated with templated functors makes it impossible to achieve.
The other way around would be to use OIIO's thread-pool as main threading backend of the application but we do not want that as there is better suited threading technologies available.

I did the same with OpenEXR's thread-pool and it works well because their thread-pool API is essentially very simple with just a "addTask" function and their simple Task and TaskGroup classes.

[el3ment]

is it possible to disable threading from the python bindings?

[lgritz]

Yes:

import OpenImageIO as oiio
oiio.attribute("threads", 1)

should set the default thread pool size to 1.