Added support for clang thread-safety analysis

The annotations have been added to SDL_mutex.h and have been made public so applications can enable this for their own code.

Clang assumes that locking and unlocking can't fail, but SDL has the concept of a NULL mutex, so the mutex functions have been changed not to report errors if a mutex hasn't been initialized. We do have mutexes that might be accessed when they are NULL, notably in the event system, so this is an important change.

This commit cleans up a bunch of rare race conditions in the joystick and game controller code so now everything should be completely protected by the joystick lock.

To test this, change the compiler to "clang -Wthread-safety -Werror=thread-safety -DSDL_THREAD_SAFETY_ANALYSIS"

src/thread/n3ds/SDL_sysmutex.c

@@ -51,10 +51,10 @@ void SDL_DestroyMutex(SDL_mutex *mutex)
 }
 
 /* Lock the mutex */
-int SDL_LockMutex(SDL_mutex *mutex)
+int SDL_LockMutex(SDL_mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS /* clang doesn't know about NULL mutexes */
 {
     if (mutex == NULL) {
-        return SDL_InvalidParamError("mutex");
+        return 0;
     }
 
     RecursiveLock_Lock(&mutex->lock);
@@ -66,17 +66,17 @@ int SDL_LockMutex(SDL_mutex *mutex)
 int SDL_TryLockMutex(SDL_mutex *mutex)
 {
     if (mutex == NULL) {
-        return SDL_InvalidParamError("mutex");
+        return 0;
     }
 
     return RecursiveLock_TryLock(&mutex->lock);
 }
 
 /* Unlock the mutex */
-int SDL_mutexV(SDL_mutex *mutex)
+int SDL_UnlockMutex(SDL_mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS /* clang doesn't know about NULL mutexes */
 {
     if (mutex == NULL) {
-        return SDL_InvalidParamError("mutex");
+        return 0;
     }
 
     RecursiveLock_Unlock(&mutex->lock);