vo: don't invoke wait, when not needed

This causes only problems, because we convert mp_time to realtime, which
is not atomic, so we introduce error. And even though on sane platforms
it should work fine, after all the sleep time is in the past.
winpthreads like to sleep for like over 10ms when the time is less than
current time, but not more than 1s.

video/out/vo.c

@@ -1150,6 +1150,9 @@ static void *vo_thread(void *ptr)
         if (vo->want_redraw) // might have been set by VOCTRLs
             wait_until = 0;
 
+        if (wait_until <= now)
+            continue;
+
         wait_vo(vo, wait_until);
     }
     forget_frames(vo); // implicitly synchronized