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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -2583,17 +2583,17 @@ _Semaphore::~_Semaphore() {
 
 void _Mutex::lock() {
 
-	mutex->lock();
+	mutex.lock();
 }
 
 Error _Mutex::try_lock() {
 
-	return mutex->try_lock();
+	return mutex.try_lock();
 }
 
 void _Mutex::unlock() {
 
-	mutex->unlock();
+	mutex.unlock();
 }
 
 void _Mutex::_bind_methods() {
@@ -2603,16 +2603,6 @@ void _Mutex::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("unlock"), &_Mutex::unlock);
 }
 
-_Mutex::_Mutex() {
-
-	mutex = Mutex::create();
-}
-
-_Mutex::~_Mutex() {
-
-	memdelete(mutex);
-}
-
 ///////////////
 
 void _Thread::_start_func(void *ud) {