Fix AnimationPlayer seeking for Discrete keys

(cherry picked from commit 7ed7ea53e6221eff6e6bf1e357706533e9a31e8c)

scene/animation/animation_player.cpp

@@ -233,7 +233,7 @@ void AnimationPlayer::_process_playback_data(PlaybackData &cd, double p_delta, f
 		pi.delta = delta;
 		pi.seeked = p_seeked;
 	}
-	pi.is_external_seeking = false;
+	pi.is_external_seeking = true; // AnimationPlayer doesn't have internal seeking.
 	pi.looped_flag = looped_flag;
 	pi.weight = p_blend;
 	make_animation_instance(cd.from->name, pi);