iconhelper: Move size computation
iconhelper: Move size computation

We do a switch on the image type, so compute the size for a certain
image type right there.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita