Deadlock fix

See comment for description. It's not the easiest thing to test, but when it wasn't working I was able to lock it within a minute or so of command+tabbing with another fullscreen app in order to occlude the PC window. Running the main thread's loop, for some reason, did not fix the issue even though it seems like it should. This changes the CVDisplayLinkStop() call to run asynchronously on the new thread