Not implemented, because I'm not sure it would fit the overall usage... but this is the code ;)

static void
murrine_draw_glow (cairo_t *cr,
                   const MurrineRGB *fill,
                   int x, int y, int w, int h)
{
	cairo_pattern_t *pat;
	MurrineRGB       glow_c;
	int              glow_r = 200; /* draw a big cirle */

	murrine_shade (fill, 1.24, &glow_c);

	cairo_rectangle (cr, x, y, w, w);
	pat = cairo_pattern_create_radial (x+w/2, y-w-glow_r, w+glow_r,
	                                   x+w/2, y-w-glow_r, w+h+glow_r);
	cairo_pattern_add_color_stop_rgba (pat, 0.0, glow_c.r, glow_c.g, glow_c.b, 0.6);
	cairo_pattern_add_color_stop_rgba (pat, 1.0, glow_c.r, glow_c.g, glow_c.b, 0.0);
	cairo_set_source (cr, pat);
	cairo_fill (cr);
	cairo_pattern_destroy (pat);
}