Modernize Mutex

- Based on C++11's `mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`
- `BinaryMutex` added for special cases as the non-recursive version
- `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`.
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.

platform/javascript/audio_driver_javascript.h

@@ -58,7 +58,7 @@ public:
 			STATE_SAMPLES_OUT,
 			STATE_MAX,
 		};
-		Mutex *mutex = nullptr;
+		Mutex mutex;
 		Thread *thread = nullptr;
 		bool quit = false;
 		int32_t state[STATE_MAX] = { 0 };