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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -33,50 +33,9 @@
 
 #include "core/os/mutex.h"
 
-class ThreadSafe {
-
-	Mutex *mutex;
-
-public:
-	inline void lock() const {
-		if (mutex) mutex->lock();
-	}
-	inline void unlock() const {
-		if (mutex) mutex->unlock();
-	}
-
-	ThreadSafe();
-	~ThreadSafe();
-};
-
-class ThreadSafeMethod {
-
-	const ThreadSafe *_ts;
-
-public:
-	ThreadSafeMethod(const ThreadSafe *p_ts) {
-
-		_ts = p_ts;
-		_ts->lock();
-	}
-
-	~ThreadSafeMethod() { _ts->unlock(); }
-};
-
-#ifndef NO_THREADS
-
-#define _THREAD_SAFE_CLASS_ ThreadSafe __thread__safe__;
-#define _THREAD_SAFE_METHOD_ ThreadSafeMethod __thread_safe_method__(&__thread__safe__);
-#define _THREAD_SAFE_LOCK_ __thread__safe__.lock();
-#define _THREAD_SAFE_UNLOCK_ __thread__safe__.unlock();
-
-#else
-
-#define _THREAD_SAFE_CLASS_
-#define _THREAD_SAFE_METHOD_
-#define _THREAD_SAFE_LOCK_
-#define _THREAD_SAFE_UNLOCK_
-
-#endif
+#define _THREAD_SAFE_CLASS_ mutable Mutex _thread_safe_;
+#define _THREAD_SAFE_METHOD_ MutexLock _thread_safe_method_(_thread_safe_);
+#define _THREAD_SAFE_LOCK_ _thread_safe_.lock();
+#define _THREAD_SAFE_UNLOCK_ _thread_safe_.unlock();
 
 #endif