Modernize Mutex

- Based on C++11's `mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`
- `BinaryMutex` added for special cases as the non-recursive version
- `MutexLock` now takes a reference. At this point the cases of null `Mutex`es are rare. If you ever need that, just don't use `MutexLock`.
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.

drivers/xaudio2/audio_driver_xaudio2.cpp

@@ -79,7 +79,6 @@ Error AudioDriverXAudio2::init() {
 	hr = xaudio->CreateSourceVoice(&source_voice, &wave_format, 0, XAUDIO2_MAX_FREQ_RATIO, &voice_callback);
 	ERR_FAIL_COND_V_MSG(hr != S_OK, ERR_UNAVAILABLE, "Error creating XAudio2 source voice. Error code: " + itos(hr) + ".");
 
-	mutex = Mutex::create();
 	thread = Thread::create(AudioDriverXAudio2::thread_func, this);
 
 	return OK;
@@ -158,15 +157,15 @@ float AudioDriverXAudio2::get_latency() {
 
 void AudioDriverXAudio2::lock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->lock();
+	mutex.lock();
 }
 void AudioDriverXAudio2::unlock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->unlock();
+	mutex.unlock();
 }
 
 void AudioDriverXAudio2::finish() {
@@ -194,14 +193,11 @@ void AudioDriverXAudio2::finish() {
 	mastering_voice->DestroyVoice();
 
 	memdelete(thread);
-	if (mutex)
-		memdelete(mutex);
 	thread = NULL;
 }
 
 AudioDriverXAudio2::AudioDriverXAudio2() :
 		thread(NULL),
-		mutex(NULL),
 		current_buffer(0) {
 	wave_format = { 0 };
 	for (int i = 0; i < AUDIO_BUFFERS; i++) {

drivers/xaudio2/audio_driver_xaudio2.h

@@ -65,7 +65,7 @@ class AudioDriverXAudio2 : public AudioDriver {
 	};
 
 	Thread *thread;
-	Mutex *mutex;
+	Mutex mutex;
 
 	int32_t *samples_in;
 	int16_t *samples_out[AUDIO_BUFFERS];