Changes :

- The page number changes when a certain amount of the previous page is
  reached. This is done in SetFirstVisPage.

- Since the page number (and pFirstVisPage) changes before the entire
  previous page disappear from VisArea, there is still a portion of it
  which is visible and must be added for paint. This is done with
  GetPrev.

Change-Id: If7704011bc11efe88321bf448d61708097334d7d
Reviewed-on: https://gerrit.libreoffice.org/15657Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>