oto: run Suspend/Resume off the calling thread on darwin

Suspend and Resume held c.cond.L to flip the toPause/toResume flags. On
iOS the audio render goroutine can hold c.cond.L for several seconds
while resume() retries transient AVAudioSession errors (e.g.
AVAudioSessionErrorCodeCannotStartPlaying), which meant the platform UI
thread could block in applicationWillResignActive / applicationDidBecome
Active for the duration of those retries.

Spawn a small goroutine to perform the cond.L acquisition and flag flip
so the caller returns immediately. The underlying audio loop, retry
policy, and error semantics are unchanged; errors from the asynchronous
transition still surface via Err on the next poll.

Updates #93

Author: hajimehoshi

driver_darwin.go

@@ -254,30 +254,36 @@ func (c *context) appendBuffer(buf32 []float32) {
 	}
 }
 
+// Suspend returns immediately. The actual AudioQueuePause runs on a
+// background goroutine so the calling thread (typically the platform UI
+// thread) is never blocked on cond.L — which can be held for several seconds
+// while resume() retries transient AVAudioSession errors. Errors from the
+// asynchronous transition surface via Err.
 func (c *context) Suspend() error {
-	c.cond.L.Lock()
-	defer c.cond.L.Unlock()
-
-	if err := c.err.Load(); err != nil {
-		return err.(error)
-	}
-	c.toPause = true
-	c.toResume = false
-	c.cond.Signal()
-	return nil
+	err := c.err.Load()
+	go func() {
+		c.cond.L.Lock()
+		defer c.cond.L.Unlock()
+		c.toPause = true
+		c.toResume = false
+		c.cond.Signal()
+	}()
+	return err
 }
 
+// Resume returns immediately. See Suspend for the rationale; AudioQueueStart
+// (with retries on transient AVAudioSession errors on iOS) runs on a
+// background goroutine.
 func (c *context) Resume() error {
-	c.cond.L.Lock()
-	defer c.cond.L.Unlock()
-
-	if err := c.err.Load(); err != nil {
-		return err.(error)
-	}
-	c.toPause = false
-	c.toResume = true
-	c.cond.Signal()
-	return nil
+	err := c.err.Load()
+	go func() {
+		c.cond.L.Lock()
+		defer c.cond.L.Unlock()
+		c.toPause = false
+		c.toResume = true
+		c.cond.Signal()
+	}()
+	return err
 }
 
 func (c *context) pause() error {