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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -34,14 +34,12 @@
 
 void CommandQueueMT::lock() {
 
-	if (mutex)
-		mutex->lock();
+	mutex.lock();
 }
 
 void CommandQueueMT::unlock() {
 
-	if (mutex)
-		mutex->unlock();
+	mutex.unlock();
 }
 
 void CommandQueueMT::wait_for_flush() {
@@ -106,7 +104,6 @@ CommandQueueMT::CommandQueueMT(bool p_sync) {
 	read_ptr = 0;
 	write_ptr = 0;
 	dealloc_ptr = 0;
-	mutex = Mutex::create();
 	command_mem = (uint8_t *)memalloc(COMMAND_MEM_SIZE);
 
 	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
@@ -124,7 +121,6 @@ CommandQueueMT::~CommandQueueMT() {
 
 	if (sync)
 		memdelete(sync);
-	memdelete(mutex);
 	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
 
 		memdelete(sync_sems[i].sem);