Modernize atomics

- Based on C++11's `atomic`
- Reworked `SafeRefCount` (based on the rewrite by @hpvb)
- Replaced free atomic functions by the new `SafeNumeric<T>`
- Replaced wrong cases of `volatile bool` by the new `SafeFlag`
- Platform-specific implementations no longer needed

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>

editor/audio_stream_preview.h

@@ -32,6 +32,7 @@
 #define AUDIO_STREAM_PREVIEW_H
 
 #include "core/os/thread.h"
+#include "core/templates/safe_refcount.h"
 #include "scene/main/node.h"
 #include "servers/audio/audio_stream.h"
 
@@ -60,9 +61,20 @@ class AudioStreamPreviewGenerator : public Node {
 		Ref<AudioStreamPreview> preview;
 		Ref<AudioStream> base_stream;
 		Ref<AudioStreamPlayback> playback;
-		volatile bool generating = false;
+		SafeFlag generating;
 		ObjectID id;
 		Thread *thread = nullptr;
+
+		// Needed for the bookkeeping of the Map
+		Preview &operator=(const Preview &p_rhs) {
+			preview = p_rhs.preview;
+			base_stream = p_rhs.base_stream;
+			playback = p_rhs.playback;
+			generating.set_to(generating.is_set());
+			id = p_rhs.id;
+			thread = p_rhs.thread;
+			return *this;
+		}
 	};
 
 	Map<ObjectID, Preview> previews;