Assert non-recursive usage of mutexes

app/src/util/thread.c

@@ -43,6 +43,8 @@ sc_mutex_destroy(sc_mutex *mutex) {
 
 void
 sc_mutex_lock(sc_mutex *mutex) {
+    // SDL mutexes are recursive, but we don't want to use recursive mutexes
+    assert(!sc_mutex_held(mutex));
     int r = SDL_LockMutex(mutex->mutex);
 #ifndef NDEBUG
     if (r) {