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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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

@@ -35,6 +35,7 @@
 #include "core/io/resource_saver.h"
 #include "core/oa_hash_map.h"
 #include "core/ordered_hash_map.h"
+#include "core/os/mutex.h"
 #include "core/os/thread_safe.h"
 #include "core/resource.h"
 #include "core/script_language.h"
@@ -44,10 +45,6 @@
 #include "modules/gdnative/gdnative.h"
 #include <nativescript/godot_nativescript.h>
 
-#ifndef NO_THREADS
-#include "core/os/mutex.h"
-#endif
-
 struct NativeScriptDesc {
 
 	struct Method {
@@ -127,9 +124,7 @@ class NativeScript : public Script {
 	String script_class_name;
 	String script_class_icon_path;
 
-#ifndef NO_THREADS
-	Mutex *owners_lock;
-#endif
+	Mutex owners_lock;
 	Set<Object *> instance_owners;
 
 protected:
@@ -266,9 +261,8 @@ private:
 
 	void _unload_stuff(bool p_reload = false);
 
+	Mutex mutex;
 #ifndef NO_THREADS
-	Mutex *mutex;
-
 	Set<Ref<GDNativeLibrary> > libs_to_init;
 	Set<NativeScript *> scripts_to_register;
 	volatile bool has_objects_to_register; // so that we don't lock mutex every frame - it's rarely needed