Memory leaking at program exit

[stechyo]

WARNING: ObjectDB instances leaked at exit (run with --verbose for details).
     at: cleanup (core/object/object.cpp:2208)
Leaked instance: :105003360452765
Leaked instance: AudioStreamPlaybackMP3:-9222368182738616162
Leaked instance: AudioStreamMP3:-9222476484633951005 - Resource path: res://sound/2.mp3
Leaked instance: :-9222377528587451125
Leaked instance: :-9222376978831637123
Leaked instance: :-9222376429075823234
Leaked instance: AudioStreamPlaybackMP3:-9222368732494428801
Leaked instance: AudioStreamPlaybackMP3:-9222367632982801024
Hint: Leaked instances typically happen when nodes are removed from the scene tree (with `remove_child()`) but not freed (with `free()` or `queue_free()`).

We seem to be leaking the stream/playbacks somehow on the demo scene. I believe hand-freeing them on the SteamAudioStreamPlayback destructor leads to a double-free crash, so we'll have to do some investigation. This shouldn't greatly impact the extension for now however.