The workers run in separate threads meaning if any significant change is made
(e.g. parameters changing, or even closing the program), these workers may still
be mid-render. This is particularly problematic when closing since the nodes a
worker is rendering may be deleted mid-render. Render backends now have a
function that pauses the main thread (but starts a second event loop so the UI
isn't frozen) until the worker threads are all finished. This way, massive
changes can be made safely without race conditions.
itsmattkc