Fix games no longer shutting down.

In the old test, s_FifoShuttingDown was stuck once it went on.  Now, the
code that sets it also flags the events to break any existing waits, but
if a new EFB/perf request comes from the CPU after that's been done,
it'll happily reset the event and wait for a no-longer-responding GPU.

Still beats manual condition variables.  I think.  Especially on Windows.