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.

core/command_queue_mt.cpp

@@ -35,14 +35,12 @@
 
 void CommandQueueMT::lock() {
 
-	if (mutex)
-		mutex->lock();
+	mutex.lock();
 }
 
 void CommandQueueMT::unlock() {
 
-	if (mutex)
-		mutex->unlock();
+	mutex.unlock();
 }
 
 void CommandQueueMT::wait_for_flush() {
@@ -107,7 +105,6 @@ CommandQueueMT::CommandQueueMT(bool p_sync) {
 	read_ptr_and_epoch = 0;
 	write_ptr_and_epoch = 0;
 	dealloc_ptr = 0;
-	mutex = Mutex::create();
 
 	command_mem_size = GLOBAL_DEF_RST("memory/limits/command_queue/multithreading_queue_size_kb", DEFAULT_COMMAND_MEM_SIZE_KB);
 	ProjectSettings::get_singleton()->set_custom_property_info("memory/limits/command_queue/multithreading_queue_size_kb", PropertyInfo(Variant::INT, "memory/limits/command_queue/multithreading_queue_size_kb", PROPERTY_HINT_RANGE, "1,4096,1,or_greater"));
@@ -130,7 +127,6 @@ CommandQueueMT::~CommandQueueMT() {
 
 	if (sync)
 		memdelete(sync);
-	memdelete(mutex);
 	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
 
 		memdelete(sync_sems[i].sem);