Expose mutex assertions

Add a function to assert that the mutex is held (or not).

app/src/util/thread.h

@@ -20,6 +20,9 @@ typedef struct sc_thread {
 
 typedef struct sc_mutex {
     SDL_mutex *mutex;
+#ifndef NDEBUG
+    sc_thread_id locker;
+#endif
 } sc_mutex;
 
 typedef struct sc_cond {
@@ -48,6 +51,14 @@ sc_mutex_unlock(sc_mutex *mutex);
 sc_thread_id
 sc_thread_get_id(void);
 
+#ifndef NDEBUG
+bool
+sc_mutex_held(struct sc_mutex *mutex);
+# define sc_mutex_assert(mutex) assert(sc_mutex_held(mutex))
+#else
+# define sc_mutex_assert(mutex)
+#endif
+
 bool
 sc_cond_init(sc_cond *cond);