kcms/fonts: Fix font hinting preview

In `combineImages`, the width and the height were calculated
incorrectly. This makes the preview image correctly obey the device
pixel ratio of the primary screen, so the preview image can be clear
and sharp.

BUG: 413673
FIXED-IN: 5.25.1

(cherry picked from commit b96d56ae)
2 jobs for Plasma/5.25 in 8 minutes and 1 second (queued for 3 seconds)