xdg-shell: ack_configure with unmapped surface

The discussion in !188 (closed) hasn't settled, opening an issue to track it.

1. Start with a fully initialized mapped toplevel
2. Compositor sends configure
3. Client sends ack_configure, followed by attach(NULL) and commit

(Alternatively, replace step 3 with: attach(NULL), commit, ack_configure, commit.)

wlroots refuses this ack_configure, but other compositors ignore it. The rationale is that the ack_configure is nonsensical.