Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

drivers/xaudio2/audio_driver_xaudio2.h

@@ -62,7 +62,7 @@ class AudioDriverXAudio2 : public AudioDriver {
 		void STDMETHODCALLTYPE OnVoiceError(void *pBufferContext, HRESULT Error) {}
 	};
 
-	Thread *thread = nullptr;
+	Thread thread;
 	Mutex mutex;
 
 	int32_t *samples_in = nullptr;