Added generator audio stream, and spectrum analyzer audio effect

Made AudioFrame and Vector2 equivalent for casting.
Added ability to obtain the playback object from stream players.
Added ability to obtain effect instance from audio server.

servers/audio_server.h

@@ -322,6 +322,7 @@ public:
 
 	int get_bus_effect_count(int p_bus);
 	Ref<AudioEffect> get_bus_effect(int p_bus, int p_effect);
+	Ref<AudioEffectInstance> get_bus_effect_instance(int p_bus, int p_effect, int p_channel = 0);
 
 	void swap_bus_effects(int p_bus, int p_effect, int p_by_effect);