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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -33,6 +33,7 @@
 #include <mono/metadata/exception.h>
 
 #include "core/os/dir_access.h"
+#include "core/os/mutex.h"
 #include "core/os/os.h"
 #include "core/project_settings.h"
 #include "core/reference.h"
@@ -43,7 +44,6 @@
 
 #include "../csharp_script.h"
 #include "../utils/macros.h"
-#include "../utils/mutex_utils.h"
 #include "gd_mono.h"
 #include "gd_mono_cache.h"
 #include "gd_mono_class.h"
@@ -74,7 +74,7 @@ MonoObject *unmanaged_get_managed(Object *unmanaged) {
 	CSharpScriptBinding &script_binding = ((Map<Object *, CSharpScriptBinding>::Element *)data)->value();
 
 	if (!script_binding.inited) {
-		SCOPED_MUTEX_LOCK(CSharpLanguage::get_singleton()->get_language_bind_mutex());
+		MutexLock lock(CSharpLanguage::get_singleton()->get_language_bind_mutex());
 
 		if (!script_binding.inited) { // Other thread may have set it up
 			// Already had a binding that needs to be setup