[x11] Resize the shape helper window to correct size
Summary:
The frame geometry doesn't necessarily correspond to the server-side
geometry of the toplevel window, we need to use the buffer geometry
instead.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D25730