Summary:
Used by clients to say where their true window geometry is relative to
the buffer size. Important if a client has client side shadows which
shouldn't be included for positioning tasks.

Technically this should be double buffered, but for consistency with all
other XdgShell properties this will be a task left to ShellClient.

Test Plan: Unit test

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D20595