A Pango::Layout created by Gtk::Widget::get_pango_context ()
does not have any font set. The Font is inherited from the
layout's context.
The actual font is set when the layout is used in
on_expose_event() via get_window ()->create_cairo_context ().
In some cases we rely on the font-size to calculate the widget's
size before rendering. At this point in time there is no drawing
context from which to inherit the style.
Furthermore, even after a call to `ensure_style()` in the c'tor
of a Widget get_font() simply returns the default value.
All Widgets that rely an Pango::Layout font size during
size-requests have to explicitly set the font.
This fixes various overlarge buttons with the initial default layout.
2abf9c66a4