Modernize Mutex

- Based on C++11's `mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`
- `BinaryMutex` added for special cases as the non-recursive version
- `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`.
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.

platform/javascript/audio_driver_javascript.cpp

@@ -267,20 +267,15 @@ int AudioDriverJavaScript::WorkletNode::create(int p_buffer_size, int p_channels
 
 void AudioDriverJavaScript::WorkletNode::start(float *p_out_buf, int p_out_buf_size, float *p_in_buf, int p_in_buf_size) {
 	godot_audio_worklet_start(p_in_buf, p_in_buf_size, p_out_buf, p_out_buf_size, state);
-	mutex = Mutex::create();
 	thread = Thread::create(_audio_thread_func, this);
 }
 
 void AudioDriverJavaScript::WorkletNode::lock() {
-	if (mutex) {
-		mutex->lock();
-	}
+	mutex.lock();
 }
 
 void AudioDriverJavaScript::WorkletNode::unlock() {
-	if (mutex) {
-		mutex->unlock();
-	}
+	mutex.unlock();
 }
 
 void AudioDriverJavaScript::WorkletNode::finish() {
@@ -288,7 +283,5 @@ void AudioDriverJavaScript::WorkletNode::finish() {
 	Thread::wait_to_finish(thread);
 	memdelete(thread);
 	thread = nullptr;
-	memdelete(mutex);
-	mutex = nullptr;
 }
 #endif