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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

editor/fileserver/editor_file_server.cpp

@@ -42,9 +42,10 @@
 void EditorFileServer::_close_client(ClientData *cd) {
 
 	cd->connection->disconnect_from_host();
-	cd->efs->wait_mutex->lock();
-	cd->efs->to_wait.insert(cd->thread);
-	cd->efs->wait_mutex->unlock();
+	{
+		MutexLock lock(cd->efs->wait_mutex);
+		cd->efs->to_wait.insert(cd->thread);
+	}
 	while (cd->files.size()) {
 		memdelete(cd->files.front()->get());
 		cd->files.erase(cd->files.front());
@@ -295,16 +296,16 @@ void EditorFileServer::_thread_start(void *s) {
 			}
 		}
 
-		self->wait_mutex->lock();
+		self->wait_mutex.lock();
 		while (self->to_wait.size()) {
 			Thread *w = self->to_wait.front()->get();
 			self->to_wait.erase(w);
-			self->wait_mutex->unlock();
+			self->wait_mutex.unlock();
 			Thread::wait_to_finish(w);
 			memdelete(w);
-			self->wait_mutex->lock();
+			self->wait_mutex.lock();
 		}
-		self->wait_mutex->unlock();
+		self->wait_mutex.unlock();
 
 		OS::get_singleton()->delay_usec(100000);
 	}
@@ -331,7 +332,6 @@ void EditorFileServer::stop() {
 EditorFileServer::EditorFileServer() {
 
 	server.instance();
-	wait_mutex = Mutex::create();
 	quit = false;
 	active = false;
 	cmd = CMD_NONE;
@@ -346,5 +346,4 @@ EditorFileServer::~EditorFileServer() {
 	quit = true;
 	Thread::wait_to_finish(thread);
 	memdelete(thread);
-	memdelete(wait_mutex);
 }