LibPDF: Parse nested Page Tree structures

We now follow nested page tree nodes to find all of the actual
page dicts, whereas previously we just assumed the root level
page tree node contained all of the page children directly.

Userland/Libraries/LibPDF/Parser.h

@@ -29,6 +29,8 @@ public:
 
     NonnullRefPtr<IndirectValue> parse_indirect_value_at_offset(size_t offset);
 
+    RefPtr<DictObject> conditionally_parse_page_tree_node_at_offset(size_t offset);
+
 private:
     bool parse_header();
     XRefTable parse_xref_table();