By default buttons fade from the hover to the normal state, by animating
the opacity of a copy of the previous border-image. This works as
expected for opaque and fully transparent pixels, but results in a
flickering effect for others.
Making StButton's fade effect work with partly transparent pixels is
hard, not using images with transparency is easy ...
https://bugzilla.gnome.org/show_bug.cgi?id=607872
