x: raise the stack decoration above the stack windows (reduces flickering)

This reduces flickering when opening new windows in a stack, see the comment.

src/render.c

@@ -344,6 +344,12 @@ void render_con(Con *con, bool render_fullscreen) {
              * aswell. */
             render_con(child, false);
         }
+
+        /* Raise the stack con itself. This will put the stack decoration on
+         * top of every stack window. That way, when a new window is opened in
+         * the stack, the old window will not obscure part of the decoration
+         * (it’s unmapped afterwards). */
+        x_raise_con(con);
     }
     }