WINGs/wview_wl: don't redisplay from wl_map if parent lacks backing

During W_RealizeView recursion, children are realized and mapped
before their parent's backing pixman image exists. wl_map triggered
wl_redisplay which sent an expose event, the widget painted itself,
and render_wl_pixmap_copy tried to composite up through the parent
chain — hitting a NULL pixman image in pixman_image_composite32.

Fix: before triggering wl_redisplay from wl_map, walk up the parent
chain and verify all ancestors have valid backings. If any ancestor
lacks a backing (still being realized), skip the redisplay — the
parent's wl_map will handle it once the full tree is ready.

WINGs/wview_wl.c

@@ -268,6 +268,15 @@ static void wl_map(void *vctx, W_View *view)
 	(void)vctx;
 	view->flags.mapped = 1;
 	if (view->flags.realized && wl_redisplay_depth == 0) {
+		/* Only redisplay if all ancestors have backings (are realized).
+		 * During W_RealizeView recursion, children may be mapped before
+		 * their parent's backing exists. */
+		W_View *v = view;
+		while (v->parent) {
+			if (!W_GetViewBacking(v->parent->window))
+				return;
+			v = v->parent;
+		}
 		wl_redisplay(vctx, view);
 	}
 }