xdg: fix de-synced SSD when shrinking Thunderbird window

xdg: fix de-synced SSD when shrinking Thunderbird window

On a slow mechine or heavy load, we sometimes see de-synced SSD with
Thunderbird windows when shrinking it. Here's how it happened:

1. Labwc sends a configure event.
2. The configure event timeouts and labwc resets view->{pending,current}
   with the old geometry.
3. Thunderbird updates the toplevel window geometry and the toplevel
   surface size, but not the subsurface size. Then it sends a commit.
4. Since now the committed window geometry and view->pending are
   different, the workaround for Qt apps is applied and the whole surface
   extent is set to view->current. Thus SSD stays in the old geometry.
5. Thunderbird finally updates the subsurface size.

So, this commit fixes this conflict between Thunderbird and the workaround
for Qt apps by using the toplevel surface size instead of the whole
surface extent to update view->current.