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- `ShaderRD` contained an `std::mutex`, which has been replaced by `Mutex`.

drivers/unix/os_unix.cpp

@@ -36,7 +36,6 @@
 #include "core/project_settings.h"
 #include "drivers/unix/dir_access_unix.h"
 #include "drivers/unix/file_access_unix.h"
-#include "drivers/unix/mutex_posix.h"
 #include "drivers/unix/net_socket_posix.h"
 #include "drivers/unix/rw_lock_posix.h"
 #include "drivers/unix/semaphore_posix.h"
@@ -123,14 +122,12 @@ void OS_Unix::initialize_core() {
 #ifdef NO_THREADS
 	ThreadDummy::make_default();
 	SemaphoreDummy::make_default();
-	MutexDummy::make_default();
 	RWLockDummy::make_default();
 #else
 	ThreadPosix::make_default();
 #if !defined(OSX_ENABLED) && !defined(IPHONE_ENABLED)
 	SemaphorePosix::make_default();
 #endif
-	MutexPosix::make_default();
 	RWLockPosix::make_default();
 #endif
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_RESOURCES);