To wake up the main thread, don't attempt to use a GCond/GMutex
(struct notify).  This kind of mixed wakeup method has known race
conditions.

The idea behind this patch is: for wakeups which happen while the main
thread is sleeping, use only a pipe.  For wakeups which happen while
the main thread is waiting for the player thread, we can later change
to GCond.  For now, accept the overhead of using a pipe for the
latter.

In the long run, the main thread will never wait for the player
thread, but will do everything asynchronously.