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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.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/io/resource_loader.h b/core/io/resource_loader.h
index 4e83427fae..172f8e979b 100644
--- a/core/io/resource_loader.h
+++ b/core/io/resource_loader.h
@@ -120,7 +120,7 @@ class ResourceLoader {
 	static ResourceLoadedCallback _loaded_callback;
 
 	static Ref<ResourceFormatLoader> _find_custom_resource_format_loader(String path);
-	static Mutex *loading_map_mutex;
+	static Mutex loading_map_mutex;
 
 	//used to track paths being loaded in a thread, avoids cyclic recursion
 	struct LoadingMapKey {