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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -406,7 +406,6 @@ Error AudioDriverWASAPI::init() {
 	exit_thread = false;
 	thread_exited = false;
 
-	mutex = Mutex::create(true);
 	thread = Thread::create(thread_func, this);
 
 	return OK;
@@ -782,14 +781,12 @@ void AudioDriverWASAPI::start() {
 
 void AudioDriverWASAPI::lock() {
 
-	if (mutex)
-		mutex->lock();
+	mutex.lock();
 }
 
 void AudioDriverWASAPI::unlock() {
 
-	if (mutex)
-		mutex->unlock();
+	mutex.unlock();
 }
 
 void AudioDriverWASAPI::finish() {
@@ -804,11 +801,6 @@ void AudioDriverWASAPI::finish() {
 
 	finish_capture_device();
 	finish_render_device();
-
-	if (mutex) {
-		memdelete(mutex);
-		mutex = NULL;
-	}
 }
 
 Error AudioDriverWASAPI::capture_start() {
@@ -863,7 +855,6 @@ String AudioDriverWASAPI::capture_get_device() {
 
 AudioDriverWASAPI::AudioDriverWASAPI() {
 
-	mutex = NULL;
 	thread = NULL;
 
 	samples_in.clear();

drivers/wasapi/audio_driver_wasapi.h

@@ -75,7 +75,7 @@ class AudioDriverWASAPI : public AudioDriver {
 	AudioDeviceWASAPI audio_input;
 	AudioDeviceWASAPI audio_output;
 
-	Mutex *mutex;
+	Mutex mutex;
 	Thread *thread;
 
 	Vector<int32_t> samples_in;