Replace delay by deadline in timedwait()

The function sc_cond_timedwait() accepted a parameter representing the
max duration to wait, because it internally uses SDL_CondWaitTimeout().

Instead, accept a deadline, to be consistent with
pthread_cond_timedwait().

app/src/util/thread.h

@@ -73,7 +73,7 @@ sc_cond_wait(sc_cond *cond, sc_mutex *mutex);
 
 // return true on signaled, false on timeout
 bool
-sc_cond_timedwait(sc_cond *cond, sc_mutex *mutex, sc_tick ms);
+sc_cond_timedwait(sc_cond *cond, sc_mutex *mutex, sc_tick deadline);
 
 void
 sc_cond_signal(sc_cond *cond);