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.
Currently, the scroll handler obeys to the page increment, but instead
of using the step increment for more precise scroll, it uses an
hardcoded one tenth of the page increment.
Use the step increment instead since it has been filled with sensible
values by ArdourFader's users.
