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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

core/io/ip.cpp

@@ -70,7 +70,7 @@ struct _IP_ResolverPrivate {
 		return IP::RESOLVER_INVALID_ID;
 	}
 
-	Mutex *mutex;
+	Mutex mutex;
 	SemaphoreOld *sem;
 
 	Thread *thread;
@@ -100,9 +100,8 @@ struct _IP_ResolverPrivate {
 
 			ipr->sem->wait();
 
-			ipr->mutex->lock();
+			MutexLock lock(ipr->mutex);
 			ipr->resolve_queues();
-			ipr->mutex->unlock();
 		}
 	}
 
@@ -115,30 +114,27 @@ struct _IP_ResolverPrivate {
 
 IP_Address IP::resolve_hostname(const String &p_hostname, IP::Type p_type) {
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
 
 	String key = _IP_ResolverPrivate::get_cache_key(p_hostname, p_type);
 	if (resolver->cache.has(key) && resolver->cache[key].is_valid()) {
 		IP_Address res = resolver->cache[key];
-		resolver->mutex->unlock();
 		return res;
 	}
 
 	IP_Address res = _resolve_hostname(p_hostname, p_type);
 	resolver->cache[key] = res;
-	resolver->mutex->unlock();
 	return res;
 }
 
 IP::ResolverID IP::resolve_hostname_queue_item(const String &p_hostname, IP::Type p_type) {
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
 
 	ResolverID id = resolver->find_empty_id();
 
 	if (id == RESOLVER_INVALID_ID) {
 		WARN_PRINT("Out of resolver queries");
-		resolver->mutex->unlock();
 		return id;
 	}
 
@@ -157,7 +153,6 @@ IP::ResolverID IP::resolve_hostname_queue_item(const String &p_hostname, IP::Typ
 			resolver->resolve_queues();
 	}
 
-	resolver->mutex->unlock();
 	return id;
 }
 
@@ -165,50 +160,43 @@ IP::ResolverStatus IP::get_resolve_item_status(ResolverID p_id) const {
 
 	ERR_FAIL_INDEX_V(p_id, IP::RESOLVER_MAX_QUERIES, IP::RESOLVER_STATUS_NONE);
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
+
 	if (resolver->queue[p_id].status == IP::RESOLVER_STATUS_NONE) {
 		ERR_PRINT("Condition status == IP::RESOLVER_STATUS_NONE");
-		resolver->mutex->unlock();
+		resolver->mutex.unlock();
 		return IP::RESOLVER_STATUS_NONE;
 	}
-	IP::ResolverStatus res = resolver->queue[p_id].status;
-
-	resolver->mutex->unlock();
-	return res;
+	return resolver->queue[p_id].status;
 }
 
 IP_Address IP::get_resolve_item_address(ResolverID p_id) const {
 
 	ERR_FAIL_INDEX_V(p_id, IP::RESOLVER_MAX_QUERIES, IP_Address());
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
 
 	if (resolver->queue[p_id].status != IP::RESOLVER_STATUS_DONE) {
 		ERR_PRINT("Resolve of '" + resolver->queue[p_id].hostname + "'' didn't complete yet.");
-		resolver->mutex->unlock();
+		resolver->mutex.unlock();
 		return IP_Address();
 	}
 
-	IP_Address res = resolver->queue[p_id].response;
-
-	resolver->mutex->unlock();
-	return res;
+	return resolver->queue[p_id].response;
 }
 
 void IP::erase_resolve_item(ResolverID p_id) {
 
 	ERR_FAIL_INDEX(p_id, IP::RESOLVER_MAX_QUERIES);
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
 
 	resolver->queue[p_id].status = IP::RESOLVER_STATUS_NONE;
-
-	resolver->mutex->unlock();
 }
 
 void IP::clear_cache(const String &p_hostname) {
 
-	resolver->mutex->lock();
+	MutexLock lock(resolver->mutex);
 
 	if (p_hostname.empty()) {
 		resolver->cache.clear();
@@ -218,8 +206,6 @@ void IP::clear_cache(const String &p_hostname) {
 		resolver->cache.erase(_IP_ResolverPrivate::get_cache_key(p_hostname, IP::TYPE_IPV6));
 		resolver->cache.erase(_IP_ResolverPrivate::get_cache_key(p_hostname, IP::TYPE_ANY));
 	}
-
-	resolver->mutex->unlock();
 }
 
 Array IP::_get_local_addresses() const {
@@ -315,7 +301,6 @@ IP::IP() {
 	singleton = this;
 	resolver = memnew(_IP_ResolverPrivate);
 	resolver->sem = NULL;
-	resolver->mutex = Mutex::create();
 
 #ifndef NO_THREADS
 
@@ -349,6 +334,5 @@ IP::~IP() {
 
 #endif
 
-	memdelete(resolver->mutex);
 	memdelete(resolver);
 }