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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

modules/mono/csharp_script.h

@@ -325,16 +325,16 @@ class CSharpLanguage : public ScriptLanguage {
 	GDMono *gdmono;
 	SelfList<CSharpScript>::List script_list;
 
-	Mutex *script_instances_mutex;
-	Mutex *script_gchandle_release_mutex;
-	Mutex *language_bind_mutex;
+	Mutex script_instances_mutex;
+	Mutex script_gchandle_release_mutex;
+	Mutex language_bind_mutex;
 
 	Map<Object *, CSharpScriptBinding> script_bindings;
 
 #ifdef DEBUG_ENABLED
 	// List of unsafe object references
 	Map<ObjectID, int> unsafe_object_references;
-	Mutex *unsafe_object_references_lock;
+	Mutex unsafe_object_references_lock;
 #endif
 
 	struct StringNameCache {
@@ -376,7 +376,7 @@ class CSharpLanguage : public ScriptLanguage {
 public:
 	StringNameCache string_names;
 
-	Mutex *get_language_bind_mutex() { return language_bind_mutex; }
+	const Mutex &get_language_bind_mutex() { return language_bind_mutex; }
 
 	_FORCE_INLINE_ int get_language_index() { return lang_idx; }
 	void set_language_index(int p_idx);