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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

1 files changed, 1 insertions, 1 deletions

diff --git a/core/string_name.h b/core/string_name.h
index 4096b8f650..0cf2f126aa 100644
--- a/core/string_name.h
+++ b/core/string_name.h
@@ -82,7 +82,7 @@ class StringName {
 	friend void register_core_types();
 	friend void unregister_core_types();
 
-	static Mutex *lock;
+	static Mutex lock;
 	static void setup();
 	static void cleanup();
 	static bool configured;