[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