Port some HiDPI and Scribble fixes to master branch

Fix 25x25 letter markers not showing up for ksudoku_scrible.svg

This is a cherry-pick of 4c38a1b4 (minus the changes already contained in 41318869), with the commit message taken and adapted from 7ea6780e.

Correct icon and text positions in game chooser for HiDPI mode

This is cherry-picked from 18c1f7f8. I noticed on vanilla master icons on the welcome screen looked the same for every game variant and were pixelated, but this might be just my local installation, and in any case is unrelated to my patch.