storyscreen: Increase bottom padding when reaching the bottom of the screen

This seems to have been an oversight on my part ages ago that only
became evident as I started to stuff text walls into storyscreens in my
campaign.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56485 75b3775d-3801-0410-9e12-e2e43121521d