x11: Update surface size for popups too

Without this, the back buffers of the wrong size
keep being used, causing flickery misdraws, as
seen when expanding the expander in the popover
in widget-factory.

gdk/x11/gdksurface-x11.c

@@ -1319,6 +1319,7 @@ x11_surface_resize (GdkSurface *surface,
       impl->unscaled_height = height * impl->surface_scale;
       surface->width = width;
       surface->height = height;
+      _gdk_surface_update_size (surface);
       _gdk_x11_surface_update_size (GDK_X11_SURFACE (surface));
     }
   else