[platforms/wayland] Always ack configure

Summary:
Per protocol a zero-size can be sent indicating that the client should resize
its surface to its liking. For now change the code such that it at least sends
the ack-configure back and leaves the current size untouched.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25128