Bug 41511

Unzooming in multiple pages mode changes the pages displayed.

To reproduce:
- open a multiple pages documents (let's say 20 pages)
- go to the page 4
- change to display multiple pages horizontally instead of just one (bottom-right, second page button next to the zoom slider)
- unzoom to display two pages (with the zoom slider) : the pages displayed on top or now the pages 7,8
- unzoom to display three pages : it is now around 10,11,12
In other words, the number of pages from the top remains unchanged (here, 4 lines of pages).

When editing a multiple pages document, one has to go back to the page one was viewing everytime the zoom changes, which is really annoying.

The proper behaviour would be to retain the focus on the page viewed (4 -> 3,4 or 4,5 -> 2,3,4 or 3,4,5 or 4,5,6, and back).

NB : this is independant of the place of the cursor.