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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -69,8 +69,6 @@ OSStatus AudioDriverCoreAudio::output_device_address_cb(AudioObjectID inObjectID
 #endif
 
 Error AudioDriverCoreAudio::init() {
-	mutex = Mutex::create();
-
 	AudioComponentDescription desc;
 	zeromem(&desc, sizeof(desc));
 	desc.componentType = kAudioUnitType_Output;
@@ -280,19 +278,15 @@ AudioDriver::SpeakerMode AudioDriverCoreAudio::get_speaker_mode() const {
 };
 
 void AudioDriverCoreAudio::lock() {
-	if (mutex)
-		mutex->lock();
+	mutex.lock();
 };
 
 void AudioDriverCoreAudio::unlock() {
-	if (mutex)
-		mutex->unlock();
+	mutex.unlock();
 };
 
 bool AudioDriverCoreAudio::try_lock() {
-	if (mutex)
-		return mutex->try_lock() == OK;
-	return true;
+	return mutex.try_lock() == OK;
 }
 
 void AudioDriverCoreAudio::finish() {
@@ -344,11 +338,6 @@ void AudioDriverCoreAudio::finish() {
 		audio_unit = NULL;
 		unlock();
 	}
-
-	if (mutex) {
-		memdelete(mutex);
-		mutex = NULL;
-	}
 }
 
 Error AudioDriverCoreAudio::capture_init() {
@@ -691,7 +680,6 @@ AudioDriverCoreAudio::AudioDriverCoreAudio() :
 		audio_unit(NULL),
 		input_unit(NULL),
 		active(false),
-		mutex(NULL),
 		device_name("Default"),
 		capture_device_name("Default"),
 		mix_rate(0),

drivers/coreaudio/audio_driver_coreaudio.h

@@ -46,7 +46,7 @@ class AudioDriverCoreAudio : public AudioDriver {
 	AudioComponentInstance input_unit;
 
 	bool active;
-	Mutex *mutex;
+	Mutex mutex;
 
 	String device_name;
 	String capture_device_name;