xdg: try to handle slow un-maximize with empty natural geometry better

xdg: try to handle slow un-maximize with empty natural geometry better

In the case of an initially-maximized view which is taking a long time
to un-maximize (seen for example with Thunderbird on slow machines), we
may end up in handle_configure_timeout() with an empty pending geometry.
In that case we have no great options (we can't center the view since we
don't know the un-maximized size yet), so set a fallback position.

v2: check wlr_box_empty() before comparing pending and current

Fixes: #2191