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>

modules/theora/video_stream_theora.h

@@ -36,6 +36,7 @@
 #include "core/os/semaphore.h"
 #include "core/os/thread.h"
 #include "core/templates/ring_buffer.h"
+#include "core/templates/safe_refcount.h"
 #include "scene/resources/video_stream.h"
 #include "servers/audio_server.h"
 
@@ -113,7 +114,7 @@ class VideoStreamPlaybackTheora : public VideoStreamPlayback {
 	bool thread_eof = false;
 	Semaphore *thread_sem;
 	Thread thread;
-	volatile bool thread_exit = false;
+	SafeFlag thread_exit;
 
 	static void _streaming_thread(void *ud);