Avoid a second rendering pass if it is not needed

[brechtm]

If the document contains no page references, a second rendering pass is not required. We can still save the .rtc though.

If a table of contents or index is present, there will always be page references though.

[brechtm]

If there are few pages references (say just the ToC and the index), only re-rendering those pages might make the second rendering pass very fast.

This requires:

• storing which pages contain page references
• saving the rendering state at the top of each page
• checking whether the rendering state for the next page is still the same after re-rendering a page

Plus this might be difficult to handle on the PDF level.