The fonts kcm offers different font rendering settings. Currently, one needs to apply the settings and
reopen the application to see the changes.
This patch adds a way to render fonts at different settings (mostly based on existing code in kfontinst) and
adds preview images to the sub-pixel and hinting comboboxes.
This is part of a planned redesign of the fonts kcm. See the discussion in https://phabricator.kde.org/T7927