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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

servers/audio_server.cpp

@@ -1137,27 +1137,28 @@ void *AudioServer::audio_data_alloc(uint32_t p_data_len, const uint8_t *p_from_d
 		copymem(ad, p_from_data, p_data_len);
 	}
 
-	audio_data_lock->lock();
-	audio_data[ad] = p_data_len;
-	audio_data_total_mem += p_data_len;
-	audio_data_max_mem = MAX(audio_data_total_mem, audio_data_max_mem);
-	audio_data_lock->unlock();
+	{
+		MutexLock lock(audio_data_lock);
+
+		audio_data[ad] = p_data_len;
+		audio_data_total_mem += p_data_len;
+		audio_data_max_mem = MAX(audio_data_total_mem, audio_data_max_mem);
+	}
 
 	return ad;
 }
 
 void AudioServer::audio_data_free(void *p_data) {
 
-	audio_data_lock->lock();
+	MutexLock lock(audio_data_lock);
+
 	if (!audio_data.has(p_data)) {
-		audio_data_lock->unlock();
 		ERR_FAIL();
 	}
 
 	audio_data_total_mem -= audio_data[p_data];
 	audio_data.erase(p_data);
 	memfree(p_data);
-	audio_data_lock->unlock();
 }
 
 size_t AudioServer::audio_data_get_total_memory_usage() const {
@@ -1399,7 +1400,6 @@ AudioServer::AudioServer() {
 	singleton = this;
 	audio_data_total_mem = 0;
 	audio_data_max_mem = 0;
-	audio_data_lock = Mutex::create();
 	mix_frames = 0;
 	channel_count = 0;
 	to_mix = 0;
@@ -1413,7 +1413,6 @@ AudioServer::AudioServer() {
 
 AudioServer::~AudioServer() {
 
-	memdelete(audio_data_lock);
 	singleton = NULL;
 }