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/wasapi/audio_driver_wasapi.cpp

@@ -397,7 +397,7 @@ Error AudioDriverWASAPI::init() {
 	exit_thread = false;
 	thread_exited = false;
 
-	thread = Thread::create(thread_func, this);
+	thread.start(thread_func, this);
 
 	return OK;
 }
@@ -767,13 +767,8 @@ void AudioDriverWASAPI::unlock() {
 }
 
 void AudioDriverWASAPI::finish() {
-	if (thread) {
-		exit_thread = true;
-		Thread::wait_to_finish(thread);
-
-		memdelete(thread);
-		thread = nullptr;
-	}
+	exit_thread = true;
+	thread.wait_to_finish();
 
 	finish_capture_device();
 	finish_render_device();

drivers/wasapi/audio_driver_wasapi.h

@@ -64,7 +64,7 @@ class AudioDriverWASAPI : public AudioDriver {
 	AudioDeviceWASAPI audio_output;
 
 	Mutex mutex;
-	Thread *thread = nullptr;
+	Thread thread;
 
 	Vector<int32_t> samples_in;