cairo draw over alpha

cr = gdk_cairo_create(GTK_WIDGET(editor->drawing_area)->window);
cairo_push_group(cr);

cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
cairo_rectangle(cr, 0.0, 0.0, (double) widget->allocation.width, (double) widget->allocation.height);
cairo_fill(cr);

/* do some other drawing */

/* draw with alpha channel */
cairo_set_source_rgba(cr, 0.25, 0.5, 1.0, 0.5);
cairo_rectangle(cr, (double) x, (double) y, (double) width, (double) height);
cairo_fill(cr);

/* pop it to source */
cairo_pop_group_to_source(cr);
cairo_paint(cr);