Fix bugs related to Fillmode and display selection in POTD
requested to merge amedeo/kdeplasma-addons:work/amedeo/potd-fillmode-and-multiscreen-fixes into master
Fillmode was not correctly applied in the wallpaper preview.
The ratio of the display did not change according to the selected display.
Those bugs were only noticeable if displays of different resolutions are available.
Also the size of the wallpaper preview was increased because it was unnecessarily small.