[wayland] Ensure that repaints_region is in frame-local coordinates

Summary:
The repaints region is in frame-local coordinates, i.e. relative to the
top-left corner of the frame geometry.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: davidedmundson, kwin

Tags: #kwin

Maniphest Tasks: T10867

Differential Revision: https://phabricator.kde.org/D24457