Summary:
It's perfectly legitimate to call setWindowSize before a buffer is
attached. This seems to have happen with plasma surfaces that commit
when attaching a shadow, but technically could happen anywhere.

By clamping to the applied surface here, we get the wrong window size
cached and not re-evaluated when a surface is eventually applied. This
leaves us thinking the windowsize is empty but with a massive margin
which actually holds the content.

We want all internal usages of xdgshellclient to use the window geometry
set. Only the wider kwin part needs to care about clamping it to the
surface.

This fixes popup placement in the plasma panel
BUG: 415317

As well as ghost notification popups with no background contrast that
you can't interact with.

Test Plan: Ran kwin

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26233