Fix #483: workaround MPV failing to begin playback of track with new sample rate on Windows

backend/playbackcommands.go

@@ -26,6 +26,8 @@ const (
 	// arg3: bool (shuffle)
 	cmdLoadItems
 	cmdLoadRadioStation // arg: *mediaprovider.RadioStation, arg2: InsertQueueMode
+
+	cmdForceRestartPlayback
 )
 
 type playbackCommand struct {
@@ -144,6 +146,13 @@ func (c *playbackCommandQueue) LoadItems(items []mediaprovider.MediaItem, insert
 	c.cmdAvailable.Signal()
 }
 
+func (c *playbackCommandQueue) addCommand(command playbackCommand) {
+	c.mutex.Lock()
+	c.queue = append(c.queue, command)
+	c.mutex.Unlock()
+	c.cmdAvailable.Signal()
+}
+
 func (c *playbackCommandQueue) filterCommandsAndAdd(excludeTypes []playbackCommandType, command playbackCommand) {
 	c.mutex.Lock()
 	j := 0