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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -148,7 +148,6 @@ Error MIDIDriverALSAMidi::open() {
 	}
 	snd_device_name_free_hint(hints);
 
-	mutex = Mutex::create();
 	exit_thread = false;
 	thread = Thread::create(MIDIDriverALSAMidi::thread_func, this);
 
@@ -165,11 +164,6 @@ void MIDIDriverALSAMidi::close() {
 		thread = NULL;
 	}
 
-	if (mutex) {
-		memdelete(mutex);
-		mutex = NULL;
-	}
-
 	for (int i = 0; i < connected_inputs.size(); i++) {
 		snd_rawmidi_t *midi_in = connected_inputs[i];
 		snd_rawmidi_close(midi_in);
@@ -179,14 +173,12 @@ void MIDIDriverALSAMidi::close() {
 
 void MIDIDriverALSAMidi::lock() const {
 
-	if (mutex)
-		mutex->lock();
+	mutex.lock();
 }
 
 void MIDIDriverALSAMidi::unlock() const {
 
-	if (mutex)
-		mutex->unlock();
+	mutex.unlock();
 }
 
 PackedStringArray MIDIDriverALSAMidi::get_connected_inputs() {
@@ -210,7 +202,6 @@ PackedStringArray MIDIDriverALSAMidi::get_connected_inputs() {
 
 MIDIDriverALSAMidi::MIDIDriverALSAMidi() {
 
-	mutex = NULL;
 	thread = NULL;
 
 	exit_thread = false;

drivers/alsamidi/midi_driver_alsamidi.h

@@ -44,7 +44,7 @@
 class MIDIDriverALSAMidi : public MIDIDriver {
 
 	Thread *thread;
-	Mutex *mutex;
+	Mutex mutex;
 
 	Vector<snd_rawmidi_t *> connected_inputs;