[wayland] Provide correct input geometry for client-side decorated clients

Summary:
Currently, the input geometry for client-side decorated clients matches
the frame geometry, which makes it impossible for a user to resize such
clients by just dragging invisible area near window borders.

BUG: 416346

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: cblack, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26716