[scenes/xrender] Correctly render client-side decorated clients

Summary:
The buffer shape is in buffer-local coordinates and must be mapped to
window coordinates. After that, we are free to map it to the global screen
coordinates.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25733