picture: use a real copy of the video format
If the format contains a palette, it may be gone after the picture is created. We want to keep it in the picture.
It might be an issue of the caller expected this behavior and was managing to palette locally. But doesn't appear to be the case. At least DVDs don't do that. Neither are the other subpicture sources setting a palette in YUVP.
Broader fix to the issue fixed by !5053 (merged)