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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

2 files changed, 3 insertions, 10 deletions

diff --git a/platform/haiku/audio_driver_media_kit.cpp b/platform/haiku/audio_driver_media_kit.cpp
index b7f6b57244..0a5df14743 100644
--- a/platform/haiku/audio_driver_media_kit.cpp
+++ b/platform/haiku/audio_driver_media_kit.cpp
@@ -67,7 +67,6 @@ Error AudioDriverMediaKit::init() {
 		ERR_FAIL_COND_V(player == NULL, ERR_CANT_OPEN);
 	}
 
-	mutex = Mutex::create();
 	player->Start();
 
 	return OK;
@@ -108,14 +107,14 @@ void AudioDriverMediaKit::lock() {
 	if (!mutex)
 		return;
 
-	mutex->lock();
+	mutex.lock();
 }
 
 void AudioDriverMediaKit::unlock() {
 	if (!mutex)
 		return;
 
-	mutex->unlock();
+	mutex.unlock();
 }
 
 void AudioDriverMediaKit::finish() {
@@ -124,15 +123,9 @@ void AudioDriverMediaKit::finish() {
 	if (samples_in) {
 		memdelete_arr(samples_in);
 	};
-
-	if (mutex) {
-		memdelete(mutex);
-		mutex = NULL;
-	}
 }
 
 AudioDriverMediaKit::AudioDriverMediaKit() {
-	mutex = NULL;
 	player = NULL;
 }
 
diff --git a/platform/haiku/audio_driver_media_kit.h b/platform/haiku/audio_driver_media_kit.h
index 06a362a89e..8272780fa7 100644
--- a/platform/haiku/audio_driver_media_kit.h
+++ b/platform/haiku/audio_driver_media_kit.h
@@ -40,7 +40,7 @@
 #include <SoundPlayer.h>
 
 class AudioDriverMediaKit : public AudioDriver {
-	Mutex *mutex;
+	Mutex mutex;
 
 	BSoundPlayer *player;
 	static int32_t *samples_in;