xwm: Fix silly region math

We were setting up the opaque region to be the window size and then
intersecting with the opaque rectange that's always contained in the window
rectangle.  Just initialize to the opaque rectangle.

src/xwayland/window-manager.c

@@ -816,14 +816,10 @@ weston_wm_window_draw_decoration(void *data)
 
 	if (window->surface) {
 		pixman_region32_fini(&window->surface->pending.opaque);
-		pixman_region32_init_rect(&window->surface->pending.opaque, 0, 0,
-					  width, height);
-
 		/* We leave an extra pixel around the X window area to
 		 * make sure we don't sample from the undefined alpha
 		 * channel when filtering. */
-		pixman_region32_intersect_rect(&window->surface->pending.opaque,
-					       &window->surface->pending.opaque,
+		pixman_region32_init_rect(&window->surface->pending.opaque, 
 					  x - 1, y - 1,
 					  window->width + 2,
 					  window->height + 2);