Function apply_rewrite 

pub fn apply_rewrite(
    edit: &mut EditDoc<'_>,
    page_ref: ObjRef,
    page_dict: &Dict,
    rewrite: &PageRewrite,
    shared: &ShareCounts,
) -> BTreeMap<usize, usize>

Apply a page’s regenerated content to edit.

page_ref names the page dictionary; page_dict is its current contents; shared names the object numbers more than one object points at.

Returns the map from each object’s old content-stream index to its new one, so the caller can renumber the page-object graph it still holds. An index missing from the map belongs to an element that was removed, and collapses to zero.