[NbtkViewport] fix adjustment updates
[NbtkViewport] fix adjustment updates

The adjustment updates need to include new page-size values. The upper
values where also incorrectly calculated.

clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx