Semaphores allow for semi-persistent signals, compared to a condition variable
which requires a mutex for proper detection. A semaphore can be 'post'ed after
writing some data on one thread, and another thread will be able to recognize
it quickly even if the post occured in between checking for data and waiting.
This more correctly fixes a race condition with events since the mixer
shouldn't be using mutexes, and arbitrary wake-ups just to make sure an event
wasn't missed was quite inefficient.
To avoid having unknown user code running in the mixer thread that could
significantly delay the mixed output, a lockless ringbuffer is used for the
mixer to provide events that a secondary thread will pop off and process.
