clients: Correct drawing glitch in stacking demo

Fix a graphics glitch in the stacking demo in which a transient
window's drop shadow is visibile within the interior of the window.

Signed-off-by: Ben Hummon <benjamin.hummon@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

clients/stacking.c

@@ -256,15 +256,11 @@ redraw_handler(struct widget *widget, void *data)
 	cairo_translate(cr, allocation.x, allocation.y);
 
 	/* Draw background. */
-	cairo_push_group(cr);
 	cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
 	set_window_background_colour(cr, window);
 	cairo_rectangle(cr, 0, 0, allocation.width, allocation.height);
 	cairo_fill(cr);
 
-	cairo_pop_group_to_source(cr);
-	cairo_paint(cr);
-
 	/* Print the instructions. */
 	cairo_move_to(cr, 5, 15);
 	cairo_set_source_rgb(cr, 1.0, 1.0, 1.0);