video: reset display_sync_error when resetting state

This would cause mpv to, in some very specific scenarios, have a
negative vsync_offset after seeking which would result in mpv requesting
a pts before the first frame to libplacebo.

Fix it by setting it to 0 when we reset state, such as after seeking.

Fixes: https://github.com/mpv-player/mpv/issues/12813

player/video.c

@@ -119,6 +119,7 @@ void reset_video_state(struct MPContext *mpctx)
     mpctx->mistimed_frames_total = 0;
     mpctx->drop_message_shown = 0;
     mpctx->display_sync_drift_dir = 0;
+    mpctx->display_sync_error = 0;
 
     mpctx->video_status = mpctx->vo_chain ? STATUS_SYNCING : STATUS_EOF;
 }