shared/cairo-util: Re-use the PangoContext for layout creation

Rather than creating a new PangoContext each time the menu redraw
handler is triggered re-use it if one was created previously.

All toytoolkit clients do create a layout (and implicitly a
PangoContext) but only those that have menu redraw
handler installed will create a new layout for each redraw of the menu,
effectively creating a new PangoContext each time.

Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>