fix pages/grid after switching single page step

[octaeder]

The PR fixes following two cases (alway start with non singlePageOffset). Left half of the images from master, right half fixed (s. details in txs window title).

1. Set non continuous, pageOffset>0. Scroll to top. Now setting singlePageStep moves pages and leaves empty grid faces at lower right of the grid:

   
   click to enlarge

   
   click to enlarge

   This issue is a direct consequence of fixing calculation of visible pages/pages list fix pdf-viewer's page display in non continuous mode #3952. This is fixed with reloadPage.

2. Set continuous, pageOffset>0 and last grid row has at most pageOffset many pages (f.ex. gridx=3 and 11 pages). Scroll to end. Then setting singlePageStep moves last pages to previous line and last grid row is empty. Have a close look at the different scrollbars to the left and right. To the left you can sroll down to the empty row. But overscroll should't be possible with continuous. Also, returning back works with fixed version: Switching to non singlePageStep brings back the row on the right side:

   
   click to enlarge

   
   click to enlarge

   This is fixd with updateScrollBars. I see no difference when removing delayedUpdate.

[sunderme]

thanks