Make --screenshot work even in (Cairo) GTK2 builds.

Make --screenshot work even in (Cairo) GTK2 builds.

I had occasion recently to want to take a puzzle screenshot on a
machine that didn't have the GTK3 libraries installed, but is advanced
enough to build the GTK2+Cairo version of the puzzles. That _ought_ to
be good enough to take screenshots using bare Cairo without GTK; I
think the only reason why I didn't bother to support it before is
because on GTK2, frontend_default_colours() queries the widget style
to choose a shade of grey. But we have a fixed fallback shade of grey
to use on GTK3, so it's easy to just use that same fallback in
headless GTK2.