Implement set_window_geometry
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