[wayland] Keep the opposite corner still when resizing a window

Summary:
According to the xdg-shell spec, configure events carry the maximum
window geometry size. If a client wants to enforce aspect ratio, it
may attach a buffer with smaller size. We need to account for that
when determining frame geometry in the commit handler.

I'm targeting 5.18 branch.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26886