Denote which PDF objects are incomplete 🔧

Many features will only be added as needed by the parent project. Thus it should be kept track of what is complete.
Also improves PDF trailer writing.

src/pdf.rs

@@ -113,9 +113,6 @@ impl<W: Write> WritePdf<Document> for W {
             root: catalog_id,
         })?;
 
-        // Write where the xref table starts
-        writer.write_start_xref()?;
-
         Ok(writer.written())
     }
 }