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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

platform/android/audio_driver_opensl.cpp

@@ -44,8 +44,8 @@ void AudioDriverOpenSL::_buffer_callback(
 
 	if (pause) {
 		mix = false;
-	} else if (mutex) {
-		mix = mutex->try_lock() == OK;
+	} else {
+		mix = mutex.try_lock() == OK;
 	}
 
 	if (mix) {
@@ -58,8 +58,8 @@ void AudioDriverOpenSL::_buffer_callback(
 		}
 	}
 
-	if (mutex && mix)
-		mutex->unlock();
+	if (mix)
+		mutex.unlock();
 
 	const int32_t *src_buff = mixdown_buffer;
 
@@ -107,7 +107,6 @@ Error AudioDriverOpenSL::init() {
 
 void AudioDriverOpenSL::start() {
 
-	mutex = Mutex::create();
 	active = false;
 
 	SLresult res;
@@ -330,13 +329,13 @@ AudioDriver::SpeakerMode AudioDriverOpenSL::get_speaker_mode() const {
 void AudioDriverOpenSL::lock() {
 
 	if (active && mutex)
-		mutex->lock();
+		mutex.lock();
 }
 
 void AudioDriverOpenSL::unlock() {
 
 	if (active && mutex)
-		mutex->unlock();
+		mutex.unlock();
 }
 
 void AudioDriverOpenSL::finish() {
@@ -359,7 +358,6 @@ void AudioDriverOpenSL::set_pause(bool p_pause) {
 
 AudioDriverOpenSL::AudioDriverOpenSL() {
 	s_ad = this;
-	mutex = Mutex::create(); //NULL;
 	pause = false;
 	active = false;
 }