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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

modules/gdnative/pluginscript/pluginscript_language.cpp

@@ -399,39 +399,18 @@ void PluginScriptLanguage::reload_tool_script(const Ref<Script> &p_script, bool
 }
 
 void PluginScriptLanguage::lock() {
-#ifndef NO_THREADS
-	if (_lock) {
-		_lock->lock();
-	}
-#endif
+	_lock.lock();
 }
 
 void PluginScriptLanguage::unlock() {
-#ifndef NO_THREADS
-	if (_lock) {
-		_lock->unlock();
-	}
-#endif
+	_lock.unlock();
 }
 
 PluginScriptLanguage::PluginScriptLanguage(const godot_pluginscript_language_desc *desc) :
 		_desc(*desc) {
 	_resource_loader = Ref<ResourceFormatLoaderPluginScript>(memnew(ResourceFormatLoaderPluginScript(this)));
 	_resource_saver = Ref<ResourceFormatSaverPluginScript>(memnew(ResourceFormatSaverPluginScript(this)));
-
-// TODO: totally remove _lock attribute if NO_THREADS is set
-#ifdef NO_THREADS
-	_lock = NULL;
-#else
-	_lock = Mutex::create();
-#endif
 }
 
 PluginScriptLanguage::~PluginScriptLanguage() {
-#ifndef NO_THREADS
-	if (_lock) {
-		memdelete(_lock);
-		_lock = NULL;
-	}
-#endif
 }