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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

drivers/pulseaudio/audio_driver_pulseaudio.cpp

@@ -291,7 +291,6 @@ Error AudioDriverPulseAudio::init() {
 
 	Error err = init_device();
 	if (err == OK) {
-		mutex = Mutex::create();
 		thread = Thread::create(AudioDriverPulseAudio::thread_func, this);
 	}
 
@@ -597,16 +596,16 @@ void AudioDriverPulseAudio::set_device(String device) {
 
 void AudioDriverPulseAudio::lock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->lock();
+	mutex.lock();
 }
 
 void AudioDriverPulseAudio::unlock() {
 
-	if (!thread || !mutex)
+	if (!thread)
 		return;
-	mutex->unlock();
+	mutex.unlock();
 }
 
 void AudioDriverPulseAudio::finish_device() {
@@ -640,10 +639,6 @@ void AudioDriverPulseAudio::finish() {
 	}
 
 	memdelete(thread);
-	if (mutex) {
-		memdelete(mutex);
-		mutex = NULL;
-	}
 
 	thread = NULL;
 }
@@ -800,7 +795,6 @@ String AudioDriverPulseAudio::capture_get_device() {
 
 AudioDriverPulseAudio::AudioDriverPulseAudio() :
 		thread(NULL),
-		mutex(NULL),
 		pa_ml(NULL),
 		pa_ctx(NULL),
 		pa_str(NULL),