output/Multiple: move Wait() to struct PlayerControl

Eliminate a dependency from MultipleOutputs on PlayerControl.

src/player/Control.hxx

@@ -280,6 +280,23 @@ struct PlayerControl {
 		CommandFinished();
 	}
 
+	/**
+	 * Checks if the size of the #MusicPipe is below the #threshold.  If
+	 * not, it attempts to synchronize with all output threads, and waits
+	 * until another #MusicChunk is finished.
+	 *
+	 * Caller must lock the mutex.
+	 *
+	 * @param threshold the maximum number of chunks in the pipe
+	 * @return true if there are less than #threshold chunks in the pipe
+	 */
+	bool WaitOutputConsumed(unsigned threshold);
+
+	bool LockWaitOutputConsumed(unsigned threshold) {
+		const ScopeLock protect(mutex);
+		return WaitOutputConsumed(threshold);
+	}
+
 private:
 	/**
 	 * Wait for the command to be finished by the player thread.