Added Source:getDuration (thanks Boolsheet!)

For streaming sources, the duration may not always be sample-accurate and may return -1 if it cannot be determined at all.

src/modules/sound/lullaby/CoreAudioDecoder.h

@@ -59,6 +59,7 @@ public:
 	bool isSeekable();
 	int getChannels() const;
 	int getBitDepth() const;
+	double getDuration();
 
 private:
 
@@ -70,6 +71,8 @@ private:
 	AudioStreamBasicDescription inputInfo;
 	AudioStreamBasicDescription outputInfo;
 
+	double duration;
+
 }; // CoreAudioDecoder
 
 } // lullaby