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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

servers/visual/visual_server_wrap_mt.cpp

@@ -180,7 +180,6 @@ VisualServerWrapMT::VisualServerWrapMT(VisualServer *p_contained, bool p_create_
 	thread = NULL;
 	draw_pending = 0;
 	draw_thread_up = false;
-	alloc_mutex = Mutex::create();
 	pool_max_size = GLOBAL_GET("memory/limits/multithreaded_server/rid_pool_prealloc");
 
 	if (!p_create_thread) {
@@ -193,6 +192,5 @@ VisualServerWrapMT::VisualServerWrapMT(VisualServer *p_contained, bool p_create_
 VisualServerWrapMT::~VisualServerWrapMT() {
 
 	memdelete(visual_server);
-	memdelete(alloc_mutex);
 	//finish();
 }