vo: further reduce redundant wakeups

In display-sync mode, the core doesn't need to woken up every vsync, but
only every time a new actual video frame needs to be queued. So don't
wake up if there are still frames to repeat.

In audio-sync mode, the wakeup is simply redundant, since there's a
separate timer (in->wakeup_pts) to control when to queue a new frame. I
think.

This finally brings the required playloop iterations down to almost the
number of video frames. (As originally intended, really.)

Also a fairly risky change.

video/out/vo.c

@@ -938,8 +938,14 @@ static bool render_frame(struct vo *vo)
         in->rendering = true;
         in->hasframe_rendered = true;
         int64_t prev_drop_count = vo->in->drop_count;
+        // Can the core queue new video now? Non-display-sync uses a separate
+        // timer instead.
+        bool can_queue =
+            !in->frame_queued && in->current_frame->num_vsyncs < 1 && use_vsync;
         pthread_mutex_unlock(&in->lock);
-        wakeup_core(vo); // core can queue new video now
+
+        if (can_queue)
+            wakeup_core(vo);
 
         stats_time_start(in->stats, "video-draw");