window: create Cairo surfaces on demand for redraw

window: create Cairo surfaces on demand for redraw

This introduces the function widget_cairo_create().

Instead of directly referencing surface->cairo_surface, use the function
widget_cairo_create(), which will create the cairo_surface as necessary,
and just returns a Cairo drawing context. Also fix window_get_surface()
similarly.

Now we can go through idle_redraw() without always creating Cairo
surfaces and committing them. This will be useful with sub-surfaces,
where repainting one sub-surface does not need to force the repaint of
all surfaces of a window.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>