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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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/servers/physics_2d/physics_2d_server_wrap_mt.h b/servers/physics_2d/physics_2d_server_wrap_mt.h
index 9a01344390..0648b8651f 100644
--- a/servers/physics_2d/physics_2d_server_wrap_mt.h
+++ b/servers/physics_2d/physics_2d_server_wrap_mt.h
@@ -67,7 +67,7 @@ class Physics2DServerWrapMT : public Physics2DServer {
 
 	bool first_frame;
 
-	Mutex *alloc_mutex;
+	Mutex alloc_mutex;
 	int pool_max_size;
 
 public: