[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