Fix is_playing() method in AnimatedSprite3D

(cherry picked from commit 2450173495b6766cb1a2015b83d4cc9f17b17af5)

scene/3d/sprite_3d.cpp

@@ -1131,7 +1131,7 @@ void AnimatedSprite3D::stop() {
 }
 
 bool AnimatedSprite3D::is_playing() const {
-	return is_processing();
+	return playing;
 }
 
 void AnimatedSprite3D::_reset_timeout() {