Fix some display bugs related to end of paragraph markers

Fix some display bugs related to end of paragraph markers

There are two regressions that are fixed here:

 * empty rows at the end of a paragraph (think after newline at end of
   paragraph or empty line in Verbatim) do not have an end-of-par
   marker. This is fixed by removing the early return in breakRow and
   letting the whole function be executed. This requires to relax an
   assertion in Paragraph::fontSpan. It makes sense here to query
   position at the end of the paragraph.

 * a newline at the end of a paragraph will be followed by and
   end-of-par marker. This is fixed by skipping the end-of-par marker
   when a new row has been requested.