player/video: only set a/v speed factor to 1 when exiting display-sync

Previously this would be called on every frame presented, this only
needs to be called if display sync was active for the last frame but
isn't active anymore.

This hid the bug mentioned in #16947 if VO was active.

Author: llyyr

player/video.c

@@ -955,9 +955,11 @@ static void schedule_frame(struct MPContext *mpctx, struct vo_frame *frame)
     }
 
     if (!mpctx->display_sync_active) {
-        mpctx->speed_factor_a = 1.0;
-        mpctx->speed_factor_v = 1.0;
-        update_playback_speed(mpctx);
+        if (mpctx->num_past_frames > 1 && mpctx->past_frames[1].num_vsyncs >= 0) {
+            mpctx->speed_factor_a = 1.0;
+            mpctx->speed_factor_v = 1.0;
+            update_playback_speed(mpctx);
+        }
 
         update_av_diff(mpctx, mpctx->time_frame > 0 ?
             mpctx->time_frame * mpctx->video_speed : 0);