Qt: Sub-pixel font rendering is gone with transparent background

Subpixel font rendering needs to know the background in order to work. This is not technically possible when we want a background blur effect, because the system compositor is responsible for any backdrop effect. Qt can not be aware of what is beneath the Quick Window.

This means that, for users who prefer subpixel font rendering over background effects, we should add an option to disable the effect so that subpixel rendering can work.