[wayland] WlShell cache the pre-maximised geometry before going fullscreen
Summary:
In shell surface, maximise mode and fullscreen are exclusive
fullscreen->toplevel should restore the state we had before maximising
Test Plan: testMaximisedToFullscreen unit test
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D16921