fix pdf-viewer scroll bar

[octaeder]

This fixes following (and may also resolve #2463):

1. Start windowed viewer showing a pdf with 5 or more pages
2. Set checked 100%
3. Clear continuous
4. Set checked fit to width
5. Choose window size such that no vertical scroll bar is necessary:
   
6. Set checked continuous, as expected a vert. scroll bar appears:
   
7. Clear continuous, other than expected the scroll bar isn’t removed:
   
8. Clear fit to width, size doesn’t fit exactly (but as expected):
   
9. Set checked fit to width, now result is as expected in 7.:
   

Repeating the procedure from 1. to 8. but using fit to window instead of fit to width is even worth:
At 6., 7. and 8. no vert. scroll bar appears (but size always fits). Skip 9.

10. Set checked continuous, scroll bar appears:
    
11. Set checked fit to window, some resizing happens:
    
12. Clear continuous, other than expected the scroll bar isn’t removed:
    
13. Same as 8.
14. Same as 9.

[sunderme]

thanks