mutablePage method
Implementation
Future<UPdfDict?> mutablePage(int index) async {
final UPdfPage? page = await document.page(index);
final UPdfRef? ref = page?.ref;
if (page == null || ref == null) return null;
final Object? staged = _changed[ref.number];
if (staged is UPdfDict) return staged;
final UPdfDict copy = page.dict.copy();
put(ref.number, copy);
return copy;
}