Adjust scene for client-side decorated clients

Summary:
Currently our Scene is quite naive about geometry. It assumes that the
window frame wraps the attached buffer/client. While this is true for X11
clients, such geometry model is not suitable for client-side decorated
clients, in our case for xdg-shell clients that set window geometry
other than the bounding rectangle of the main surface.

In general, the proposed solution doesn't make any concrete assumptions
about the order between frame and buffer geometry, however we may still
need to reconsider the design of Scene once it starts to generate quads
for sub-surfaces.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, romangg, kwin

Tags: #kwin

Maniphest Tasks: T10867

Differential Revision: https://phabricator.kde.org/D24462