[server] Introduce SurfaceInterface::boundingRect()

Summary:
The new method provides a convenient way for determining the rectangle
that bounds the given surface and all of its sub-surfaces. This can be
very handy when determining the effective window geometry.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27828