Reimplement Mutex with C++'s <mutex>

Main:
- It's now implemented thanks to `<mutex>`. No more platform-specific implementations.
- `BinaryMutex` (non-recursive) is added, as an alternative for special cases.
- Doesn't need allocation/deallocation anymore. It can live in the stack and be part of other classes.
- Because of that, it's methods are now `const` and the inner mutex is `mutable` so it can be easily used in `const` contexts.
- A no-op implementation is provided if `NO_THREADS` is defined. No more need to add `#ifdef NO_THREADS` just for this.
- `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`.
- Thread-safe utilities are therefore simpler now.

Misc.:
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.
- Every case of lock, do-something, unlock is replaced by `MutexLock` (complex cases where it's not straightfoward are kept as as explicit lock and unlock).
- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

servers/audio/audio_driver_dummy.cpp

@@ -49,7 +49,6 @@ Error AudioDriverDummy::init() {
 
 	samples_in = memnew_arr(int32_t, buffer_frames * channels);
 
-	mutex = Mutex::create();
 	thread = Thread::create(AudioDriverDummy::thread_func, this);
 
 	return OK;
@@ -95,16 +94,16 @@ AudioDriver::SpeakerMode AudioDriverDummy::get_speaker_mode() const {
 
 void AudioDriverDummy::lock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->lock();
+	mutex.lock();
 };
 
 void AudioDriverDummy::unlock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->unlock();
+	mutex.unlock();
 };
 
 void AudioDriverDummy::finish() {
@@ -120,14 +119,11 @@ void AudioDriverDummy::finish() {
 	};
 
 	memdelete(thread);
-	if (mutex)
-		memdelete(mutex);
 	thread = NULL;
 };
 
 AudioDriverDummy::AudioDriverDummy() {
 
-	mutex = NULL;
 	thread = NULL;
 };
 

servers/audio/audio_driver_dummy.h

@@ -39,7 +39,7 @@
 class AudioDriverDummy : public AudioDriver {
 
 	Thread *thread;
-	Mutex *mutex;
+	Mutex mutex;
 
 	int32_t *samples_in;